Developing device using a developing liquid and image forming apparatus including the same

ABSTRACT

A developing device for developing a latent image formed on an image carrier with a developing liquid includes a liquid storing section for storing the liquid consisting of a developing substance and a carrier liquid. A developer carrier deposits the liquid fed from the liquid storing section thereon. A float type liquid level sensor senses a liquid level in the liquid storing section. The liquid level sensor includes a magnetic force generating member for generating a magnetic force, a magnetic force sensing device for sensing the magnetic force, and floats movable up and down in accordance with the liquid level in the liquid storing section. A distance between the magnetic force generating member and the magnetic force sensing device varies in accordance with the movement of the floats, allowing the liquid level to be determined on the basis of the output of the magnetic force sensing device. An image forming apparatus including the developing device is also disclosed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a developing device fordeveloping a latent image formed on an image carrier with a developingliquid consisting or toner or similar developing substance and a carrierliquid, and a copier, facsimile apparatus, printer or similar imageforming apparatus including the same. More particularly, the presentinvention relates to a developing device including a reservoir storingthe developing liquid and an agitator rotatable in the reservoir foragitating the developing liquid, and an image forming apparatusincluding the same.

[0003] 2. Description of the Background Art

[0004] A developing device of the type using a developing liquid, whichconsists of toner or similar developing substance and a carrier liquid,is conventional. This type of developing device includes a cylindricalreservoir storing the developing liquid and a developing roller orsimilar developer carrier. The developer carrier conveys the developingliquid deposited thereon to a developing position where aphotoconductive drum or similar image carrier is located. The developingliquid is transferred from the developer carrier to a latent imageformed on the image carrier at the developing position, therebydeveloping the latent image. This kind of configuration is taught in,e.g., Japanese Patent Laid-Open Publication No. 11-352783.

[0005] The developing liquid customarily stored in the reservoir isthixotropic, i.e., it has relatively high viscosity and contains thedeveloping substance by a content of 5% to 40%. The viscosity of thethixotropic liquid decreases little by little when subjected to ashearing force, but increases little by little when freed from theshearing force. An agitator implemented as a paddle is disposed in thereservoir and rotatable for agitating the developing liquid. The paddlein rotation causes the developing liquid to swirl and lower itsviscosity little by little. At the same time, the toner or similardeveloping substance (simply toner hereinafter) is evenly dispersed inthe entire developing liquid, providing the liquid with a uniform tonercontent.

[0006] A float type liquid level sensor is sometimes disposed in thereservoir for sensing a liquid level in the reservoir, so thatpreselected control is executed in accordance with the sensed liquidlevel. Typical of this type of liquid level sensor includes a magnet orsimilar magnetic force generating means, Hall elements, magneticswitches or similar magnetic force sensing device, and a float movableup and down in accordance with the liquid level. One of the magneticforce generating member and magnetic force sensing device is affixed tothe float and movable up and down together with the float while theother of them is affixed to the reservoir. A distance between themagnetic force generating member and the magnetic force sensing devicevaries due to the up-and-down movement of the float. This allows themagnetic force sensing device to sense a liquid level in the reservoir.

[0007] The problem with the conventional float type liquid level sensoris that the float is sometimes positioned at a level different from theactual liquid level in the reservoir. For example, while the top of thefloat is usually positioned above the liquid level, the developingliquid deposits on the float due to the waves and eddies of the liquid.When only the liquid evaporates or drops along the side of the float,the toner is left on the top of the float. The toner sequentiallyaccumulates on the top of the float due to repeated deposition,evaporation and drop, causing the float to sink due to the weight of thetoner. Consequently, the float is positioned at a level different fromthe actual liquid level. Particularly, the toner easily accumulates onthe top of the float when the developing liquid is thixotropic.

[0008] Further, the developing liquid in the reservoir usually flows dueto agitation during operation, making the liquid level inconstant. Morespecifically, the liquid surface noticeably waves or sequentially risesoutward from the center of agitation. When the actual level at theposition of the float differs from the actual liquid level (stillstate), the float is positioned at a level different from the actualliquid level. As a result, the magnetic generating member or themagnetic force sensing device connected to the float is, of course,located at an unexpected level, producing a difference between theactual liquid level and the sensed liquid level.

[0009] Moreover, an eddy appears at the center of the developing liquidin the cylindrical reservoir. The eddy lowers the liquid level at thecenter while raising it at the periphery. In addition, friction actingbetween the developing liquid and the wall of the reservoir generates aforce that obstructs the swirl of the liquid and thereby causes theliquid surface to wave. This also results in the difference between theactual liquid level and the sensed liquid level.

[0010] A toner content sensor, for example, is often disposed in thereservoir for sensing the toner content of the developing liquid. Inthis case, the paddle or agitator is located at a position offset fromthe center of the reservoir, so that the paddle does not contact thetoner content sensor. The paddle should therefore be short enough forlayout reasons and cannot sufficiently agitate the developing liquid inthe reservoir. Furthermore, the paddle cannot sufficiently agitate thedeveloping liquid in the up-and-down direction although it can agitateit in the horizontal direction.

[0011] In the developing device, the developing liquid is left on thedeveloper carrier after development and on the image carrier after imagetransfer. Such residual liquids should preferably be collected byrespective cleaners and returned to the reservoir to be reused. Thecollected liquids, however, often have toner contents different from theoriginal content before development and therefore cause the tonercontent of the developing liquid stored in the reservoir to vary.

[0012] To solve the above problem, an arrangement may be made such thata carrier liquid, toner or similar developing substance, developingliquid or similar control agent is replenished to the reservoir inaccordance with a toner content sensed by a toner content sensor, asproposed in the past. With this configuration, even when the collectedliquids are returned to the reservoir and vary the toner content of thedeveloping liquid in the reservoir, the control agent allows the liquidto restore its original toner content.

[0013] The thixotropic developing liquid is capable of forming an imagein a smaller amount than a developing liquid whose standard tonercontent is lower than 5%. However, it is difficult to stabilize thetoner content of the thixotropic developing liquid stored in thereservoir for the following reason.

[0014] It is a common practice with the developing device to add acontrol agent whose toner content is higher than the standard tonercontent to the developing liquid, which is diluted by the collectedliquids. So long as a traditional developing liquid whose standard tonercontent is 1% or below is used, a thixotropic developing liquid having atoner content of about 10% can rapidly restore the standard tonercontent when added as a control agent. This is because a developingliquid whose toner content is far higher than the standard toner contentcan be added as a control agent and can therefore restore the standardtoner content when added in a small amount. On the other hand, even whenthe developing liquid is thixotropic and has a toner content of 5% to40%, a control agent whose toner content is higher than the standardcontent must be added when the developing liquid is diluted. However,the control agent cannot exhibit the properties of a liquid unless thetoner content thereof is limited. It follows that the control agent mustbe replenished in a far greater amount than the traditional low tonercontent, low viscosity developing liquid. However, the amount of controlagent that can be replenished to the reservoir is limited. Even if thecontrol agent is replenished to the upper limit, then it often fails toeffectively increase the toner content of the developing liquid in thereservoir. This makes it difficult to stabilize the toner content of thedeveloper in the reservoir.

SUMMARY OF THE INVENTION

[0015] It is therefore a first object of the present invention toprovide a developing device capable of reducing a difference between anactual liquid level in a reservoir and a liquid level sensed by a floattype liquid level sensor, and an image forming apparatus including thesame.

[0016] It is a second object of the present invention to provide adeveloping device capable of reducing an error in a sensed liquid levelascribable to the waving of a developing liquid in a reservoir, and animage forming apparatus including the same.

[0017] It is a third object of the present invention to provide adeveloping device capable of causing substantially the entire developingliquid in a reservoir to swirl and reduce its viscosity in contact withan agitator even when a content sensor and other members are arrangedwithin the reservoir, and an image forming apparatus including the same.

[0018] It is a fourth object of the present invention to provide adeveloping device capable of sufficiently agitating a developing liquidin a reservoir in both of horizontal and up-and-down directions, and animage forming apparatus including the same.

[0019] It is a fifth object of the present invention to provide adeveloping device capable of stabilizing the toner content of adeveloping liquid in a reservoir more than conventional, and an imageforming apparatus including the same.

[0020] A developing device for developing a latent image formed on animage carrier with a developing liquid of the present invention includesa liquid storing section for storing the liquid consisting of adeveloping substance and a carrier liquid. A developer carrier depositsthe liquid fed from the liquid storing section thereon. A float typeliquid level sensor senses a liquid level in the liquid storing section.The liquid level sensor includes a magnetic force generating member forgenerating a magnetic force, a magnetic force sensing device for sensingthe magnetic force, and floats movable up and down in accordance withthe liquid level in the liquid storing section. A distance between themagnetic force generating member and the magnetic force sensing devicevaries in accordance with the movement of the floats, allowing theliquid level to be determined on the basis of the output of the magneticforce sensing device.

[0021] An image forming apparatus including the above developing deviceis also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

[0023]FIG. 1 is a view showing a first embodiment of an image formingapparatus in accordance with the present invention;

[0024]FIG. 2 is a plan view showing a developing unit included in thefirst embodiment;

[0025]FIG. 3 is an exploded isometric view showing a liquid adjustingsection included in the developing unit;

[0026]FIG. 4A is an isometric view showing a float type liquid levelsensor included in the liquid adjusting section;

[0027]FIG. 4B is an isometric view showing the liquid level sensortogether with a content signal outputting device;

[0028]FIGS. 5A and 5B are views demonstrating the operation of theliquid level sensor;

[0029]FIGS. 6A and 6B are isometric views showing a modification of theliquid level sensor;

[0030]FIG. 7 is a block diagram schematically showing electric circuitryincluded in the illustrative embodiment;

[0031]FIG. 8 is a flowchart demonstrating a specific operation of asensing controller included in the circuitry of FIG. 7;

[0032]FIG. 9 is a flowchart demonstrating a specific operation of areplenishment controller also included in the circuitry of FIG. 7;

[0033]FIG. 10 is an isometric view showing the liquid adjusting sectionwith a second reservoir being closed by a lid;

[0034]FIGS. 11 through 13 are plan views showing a flexible paddleincluded in the illustrative embodiment in consecutive stages;

[0035]FIG. 14 is a plan view showing a cylindrical, second reservoirtogether with a paddle;

[0036]FIG. 15 is a section showing how a developing liquid is agitatedin the second reservoir of FIG. 14;

[0037]FIG. 16 is an isometric view of a second reservoir provided with ataper at its bottom;

[0038]FIG. 17 is a plan view showing the flow of the developing liquidin the developing unit;

[0039]FIG. 18 is a side elevation showing a modification of the flexiblepaddle shown in FIG. 10;

[0040]FIG. 19 is a perspective view showing a conventional reservoir;

[0041]FIG. 20 is a section showing how a developing liquid is agitatedin the conventional reservoir;

[0042]FIG. 21 is a plan view showing a developing unit included in asecond embodiment of the present invention;

[0043]FIG. 22 is an exploded isometric view showing a liquid adjustingsection included in the developing unit of FIG. 21;

[0044]FIG. 23 is an isometric view of a reservoir included in the secondembodiment;

[0045]FIG. 24 is a view showing how a developing liquid flows when asingle non-flexible paddle is positioned in the axial direction in athird embodiment of the present invention;

[0046]FIG. 25 is a view showing how a developing liquid flows when twonon-flexible paddles are positioned one above the other in the thirdembodiment;

[0047]FIG. 26 is an isometric view showing a modification of thenon-flexible paddles;

[0048]FIG. 27 is a view showing how a developing liquid flows inaccordance with the rotation of the paddles of FIG. 26;

[0049]FIG. 28 is a view showing a fourth embodiment of the presentinvention;

[0050]FIG. 29 is a view showing a sweeping unit applicable to the fourthembodiment;

[0051]FIG. 30 is a flowchart demonstrating a specific operation of areplenishment controller included in the fourth embodiment;

[0052]FIG. 31 is a perspective view showing a blade representative of amodification of the fourth embodiment;

[0053]FIG. 32 is a section showing a roller representative of anothermodification of the fourth embodiment;

[0054]FIG. 33 is a view showing a fifth embodiment of the presentinvention;

[0055]FIG. 34 is a block diagram schematically showing electriccircuitry included in the fifth embodiment;

[0056]FIG. 35 is a view showing a modification of bypass conveying meansincluded in the fifth embodiment;

[0057]FIG. 36 is a graph showing a relation between a developing liquidbefore development and the equilibrium value of image density;

[0058]FIG. 37 is a view showing a modification of the fifth embodiment;

[0059]FIG. 38 is a view showing another modification of the fifthembodiment; and

[0060]FIG. 39 is a view showing a sixth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Preferred embodiments of the image forming apparatus inaccordance with the present invention will be described hereinafter.

First Embodiment

[0062] Referring to FIG. 1 of the drawings, an image forming apparatusembodying the present invention is shown and implemented as anelectrophotographic printer by way of example. This embodiment isdirected mainly toward the first object stated earlier. As shown, theprinter includes a photoconductive drum (drum hereinafter) 1, which is aspecific form of an image carrier. Arranged around the drum 1 are acharger 2, a developing unit or developing device 100, an intermediateimage transfer drum (intermediate drum hereinafter) 3, and a drumcleaner 4 for cleaning the drum 1. A backup roller 5 is positioned atthe right-hand side of the intermediate drum 3, as viewed in FIG. 1, andheld in contact with the drum 3 to thereby form a nip having apreselected width.

[0063] A motor or similar drive means causes the drum 1 to rotate at apreselected speed in a direction indicated by an arrow in FIG. 1(clockwise). The charger 2 uniformly charges the surface of the drum 1.An optical writing unit, not shown, scans the charged surface of thedrum 1 with a laser beam LB in accordance with image data, therebyforming a latent image on the drum 1. The developing unit 100 developsthe latent image with a developing liquid. The developed image isbrought to the position where the drum 1 contacts the intermediate drum3 in accordance with the rotation of the drum 1.

[0064] Drive means, not shown, causes the intermediate drum 3 to rotateat the same peripheral speed as the drum in a direction indicated by anarrow in FIG. 1 (counterclockwise). The developed image is transferredfrom the drum 1 to the intermediate drum 3 (primary transfer). Thedeveloped image is then conveyed to the nip between the intermediatedrum 3 and the backup roller 5.

[0065] A sheet feeder, not shown, feeds a paper sheet or similarrecording medium 6 to the nip at such a timing that the paper sheet 6meets the developed image carried on the intermediate drum 3. At thenip, the developed image is transferred from the intermediate drum 3 tothe paper sheet 6 (secondary transfer). Subsequently, a fixing unit, notshown, fixes the developed image on the paper sheet 6 with, e.g., heat.The paper sheet 6 with the fixed image is driven out of the printer.

[0066] After the primary transfer, some developing liquid is left on thedrum 1 without being transferred to the intermediate drum 3. The drumcleaner 4 mechanically scrapes off such a developing liquid with a blade4 a. A screw 4 b also included in the drum cleaner 4 conveys thedeveloping liquid removed by the blade 4 a to a collection pipe 8. Thedeveloping liquid drops in the collection pipe 8 to a second reservoir,which will be described later, due to gravity.

[0067] An intermediate drum cleaner, not shown, removes the developingliquid left on the intermediate drum 3 after the secondary transferwithout being transferred to the paper sheet 6.

[0068] A discharge lamp, not shown, discharges part of the drum 1 movedaway from the position where the drum 1 and intermediate drum 3 contacteach other, thereby preparing the drum 1 for the next image formation.

[0069] The developing unit 100 is generally made up of a developingsection 101, a collecting section or collecting means 102, a liquidadjusting section 103, and a replenishing section or content adjustingmeans 104. The developing section 101 includes a developing roller 105,a coating roller 106, a metering blade 107, a first screw 108, a secondscrew 109, and a first reservoir 110. A developing liquid (simply liquidhereinafter) 7, which is a toner and carrier liquid mixture, is storedin the first reservoir 110 in an amount of about 100 cc to 150 cc. It isto be noted that toner is representative of a developing substance. Theliquid 7 has viscosity controlled to 100 mPa·s to 10, 000 mPa·s andtoner content controlled to 5% to 40%. More specifically, in theillustrative embodiment, the liquid 7 has viscosity of about 300 mPa·sand toner content of 15%.

[0070] The coating roller 106 is positioned in the first reservoir 110above the liquid 7 such that the roller 106 does not contact the surfaceof the liquid 7 in a stand-by state. The first and second screws 108 and109 are also positioned in the first reservoir 110 in parallel to eachother. On the start of printing operation, drive means, not shown,causes the two screws 108 and 109 to rotate in opposite directions toeach other. As a result, the surface of the liquid 7 above the screws108 and 109 rises and contacts the coating roller 106. Drive means, notshown, causes the coating roller 106 to rotate counterclockwise asviewed in FIG. 1. While the coating roller 106 conveys the liquid 7deposited thereon, the metering blade 107 regulates the thickness of theliquid 7 to thereby form a thin liquid layer. Part of the thin liquidlayer is applied to the developing roller 105, which is rotating incontact with the coating roller 106, at a rate of about 30 cc for aminute. The developing roller 105 then conveys the liquid 7 to adeveloping position where the roller 105 contacts the drum 1, therebydeveloping the latent image. The liquid 7 left on the developing roller105 after the development is brought to a position where the roller 105faces the collecting section 102 in accordance with the rotation of theroller 105.

[0071] The collecting section 102 includes a collecting roller 111, ablade 112, a screw 113, and a pipe 114. The collecting roller 111rotates in contact with part of the surface of the developing roller 105moved away from the developing position and thereby collects the liquid7 left on the roller 105. The blade 112 mechanically scrapes off theliquid 7 collected by the collecting roller 111. The screw 113 conveysthe liquid 7 removed by the blade 112 to the pipe 114, which terminatesat the previously mentioned second reservoir.

[0072] The liquid adjusting section 103 includes a second reservoir 115,paddles or agitators 116 and 117, a content signal outputting device118, a pump 120, and a pipe 121. The second reservoir 115 also storesthe liquid 7 and has its open top closed by a lid 119. The contentsignal outputting device 118 constitutes toner content sensing meansresponsive to a toner content in combination with a control unit notshown.

[0073] A motor, not shown, causes the paddles 116 and 117 to rotate inthe second reservoir 115 so as to rotate, or agitate, the liquid 7 insubstantially the horizontal direction. The toner content sensing meansmentioned above senses the toner content of the liquid 7 being soagitated.

[0074] The pipe 121 is connected to the bottom of the second reservoir115 at one end and connected to a drain pipe 122 belonging to the firstreservoir 110 at the other end. The pump 120 is positioned at theintermediate portion of the pipe 121. The pump 120 delivers the liquid 7from the second reservoir 115 to the first reservoir 110. When theamount of the liquid 7 delivered from the reservoir 115 to the reservoir110 is excessive, the liquid level in the reservoir 115 rises. As aresult, excessive part of the liquid 7 overflows the reservoir 110 andreturns to the reservoir 115 via an overflow pipe not shown.

[0075] The replenishing section 104 includes a carrier bottle 123storing a carrier liquid and a liquid bottle 124 storing a developingliquid. A carrier pump 147 delivers the carrier liquid from the carrierbottle 123 to the second reservoir 115. A liquid pump 146 delivers thedeveloping liquid from the liquid bottle 124 to the second reservoir115. The control unit or control means, not shown, controls theoperation of the replenishing section 104. The developing liquid in theliquid bottle 124 has a toner content controlled to 15%, which isdesirable for development. This toner content is used as a standardtoner content in the illustrative embodiment.

[0076] Specifically, the control unit controls the liquid pump 146 andcarrier pump 147 in accordance with a toner content signal output fromthe content signal outputting device 118. The pumps 146 and 147replenish an adequate amount of developing liquid and an adequate amountof carrier liquid, respectively, to the second reservoir 115, therebycontrolling the toner content of the liquid 7 in the reservoir 115.Therefore, even when the mixture of the liquid 7 collected from thedeveloping roller 105 and the liquid 7 collected from the drum 1 has atoner content different from the original toner content of the freshliquid 7, the mixture can be returned to the second reservoir 115 andreused.

[0077] In the developing unit 100, the developing section 101 andcollecting section 102 are constructed into a single cartridge, asindicated by a dash-and-dot line in FIG. 1. The cartridge is easilyremovable from the other part of the developing unit 100 and from theprinter body at the time of repair or maintenance. A coupling 136connects the drain pipe 1232 of the cartridge to the pipe 121 of theliquid adjusting section 103.

[0078] A desirable procedure for dismounting the above cartridge fromthe printer body will be described hereinafter. First, the pump 120 ofthe liquid adjusting section 103 is driven in the reverse direction inorder to return the liquid 7 in the first reservoir 110 to the secondreservoir 115. Subsequently, after a drain valve 137 included in thedrain pipe 122 has been closed, the coupling 136 is manipulated toseparate the drain pipe 122 and pipe 121. The drain valve 137 closedbefore the separation of the drain pipe 122 and 121 prevents the liquid7 left in the first reservoir 110 and drain pipe 122 from wastefullyleaking. The pump 120 included in the pipe 121 successfully prevents theliquid 7 in the second reservoir 115 from leaking when the pipe 121 anddrain pipe 122 are separated. To help the pump 120 obviate such leakage,the coupling 136 may be provided with a valve function, if desired.

[0079] As shown in FIGS. 2 and 3, the content signal outputting device118 includes a support plate 129 protruding from the underside of thelid 119 and a photosensor 132 positioned above the lid 119. The device118 additionally includes a disk unit rotatably mounted on the supportplate 129 and a disk motor 133 for causing the disk unit to rotate.

[0080] As shown in FIG. 3, the lid 119 is formed with four circularholes 134 a, 134 b, 134 c and 134 d and a rectangular hole 134 e. Thepipe 8 of the drum cleaner 4, the pipe 114 of the collecting section102, the pipe extending from the liquid bottle 124 and the pipeextending from the carrier bottle 123 are respectively guided into thesecond reservoir 115 via the circular holes 134 a through 134 d. Theabove pipes each have an end positioned above the liquid level in thesecond reservoir 115 and cause the associated liquid to drop to theliquid level. This prevents the liquids from staying in the pipes andstopping them up.

[0081] The disk unit is made up of a pair of outer disks 131 a and 131 band an inner disk 130 sandwiched between the outer disks 131 a and 131b. The inner disk 130 is smaller in diameter than the outer disks 131 aand 131 b and rotatable about an axis offset from the axis of rotationof the outer disks 131 a and 131 b. The circumferential surface of theinner disk 130 is polished.

[0082] When the disk unit rotates with part of its circumferenceimmersed in the liquid 7, the liquid 7 fills a circumferential grooveformed between the inner disk 130 and the outer disks 131 a and 131 b.The width of the groove sequentially varies in the circumferentialdirection because the inner disk 130 is eccentric with respect to theaxis of the outer disks 131 a and 131 b. A metering blade, not shown,contacts the circumference of the outer disks 131 a and 131 b. Theliquid 7 therefore forms a film varying in width in the circumferentialdirection in part of the above groove moved away from the meteringblade.

[0083] The photosensor 132 has a light emitting element and alight-sensitive element although not shown specifically. The lightemitting element emits light toward the liquid film formed in the groovevia a hole 134 e formed in the lid 119. The polished surface of theinner disk 130, i.e., the bottom of the groove reflects the lightincident thereto via the liquid film. The reflected light is incident tothe light-sensitive element via the liquid film and hole 134 e. Thelight-sensitive element sends a signal representative of the quantity ofincident light to the control unit.

[0084] The quantity of light transmitted through the liquid film dependson the toner content of the liquid 7. However, the problem with thethixotropic liquid 7 whose toner content is high is that it causes thequantity of transmission to noticeably vary with respect to tonercontent. Specifically, if the liquid film has constant thickness, ittransmits light or does not transmit it when the toner content slightlyvaries. In light of this, the content signal outputting device 118provides the liquid film formed in the annular groove with a slope inthickness, so that the liquid film transmits the light with variouswidths. This allows the liquid film to surely transmit the light beforethe disk unit completes one rotation.

[0085] The output of the light-sensitive element continuously varies inaccordance with the rotation angle of the disk unit, i.e., the thicknessof the liquid film. The integrated output value for a single rotation ofthe disk unit corresponds to the total quantity of light incident to thelight-sensitive element and is correlated to the toner content of theliquid 7. The control unit integrates the varying output of thelight-sensitive element while the disk unit makes one rotation, andcalculates a toner content on the basis of the result of integration.

[0086] With the above-described toner content sensing means, it ispossible to determine the toner content of the liquid 7 despite that theliquid 7 is thixotropic.

[0087] The reflection type photosensor 132 may be replaced with atransmission type photosensor, if desired. The transmission typephotosensor includes one of a light emitting element and alight-sensitive element disposed in the inner disk 130, which is, inthis case, formed of glass, resin or similar transparent material. Theother of the light emitting element and light-sensitive element ispositioned outside of the disk unit. Light issuing from the lightemitting element is transmitted through the liquid film only once andincident to the light-sensitive element.

[0088] Arrangements unique to the illustrative embodiment will bedescribed hereinafter. FIG. 4A shows a float type liquid level sensor135 included in the liquid adjusting section 103. As shown, the liquidlevel sensor 135 includes two spherical floats 140, two guide rods 139guiding the floats 140 in the up-and-down direction, a ring pin 141,four Hall elements or magnetic force sensing means 143, and a magneticforce generating member 144. The ring pin 141 has a pair of ringportions 141 a. The guide rods 139 are studded on the underside of thelid 119, FIG. 1, and respectively passed through the ring portions 141a.

[0089] The floats 140 floating in the liquid 7, FIG. 1, are supported byopposite ends of the ring pin 141 and movable up and down in accordancewith the liquid level. The guide rods 139 prevent the floats 140 frombeing entrained by the liquid 7 in the direction in which the liquid 7swirls. The magnetic force generating member 144 is affixed to theintermediate portion of the ring pin 141.

[0090] The support plate 129, FIG. 3, also affixed to the underside ofthe lid 119, FIG. 1, is positioned between the two guide rods 139 andfaces the magnetic force generating member 144. The four Hall elements143 are arranged on the support plate 129 one above the other. The topHall element 143 to the bottom Hall element 143 are respectivelyresponsive to an upper-limit liquid level (level C), a standard liquidlevel (level B), a lower-limit liquid level (level A), and anemergency-stop liquid level. When the liquid level in the secondreservoir 115 falls below the lower limit, the disk unit is entirelypositioned above the liquid 7. When the liquid level rises above theupper limit, the disk unit is entirely immersed in the liquid 7. In anycase, the liquid 7 fails to form a film on the disk unit and preventstoner content from being sensed.

[0091] A voltage of, e.g., 5 V is applied to each Hall element 143. Whenthe Hall element 143 senses a magnetic force of S pole or N pole, itsends its output of 0 V to a replenishment controller, which will bedescribed later. Specifically, when the floats 140 rise or fall inaccordance with the liquid level, one of the Hall elements 140positioned at the same level as the floats 140 senses the magnetic forceof the magnetic force generating member 144.

[0092] The Hall elements 143 are not affixed to the floats 140, butaffixed to the support plate 129. It follows that wires, not shown, forfeeding power and interchanging signals can be connected to the Hallelements 143 without being extended through the liquid 7. This preventssuch wires from coiling round the floats 140, ring pin 141 and guiderods 139 despite the up-and-down movement of the floats 140.

[0093] The floats 140 should preferably be formed of polystyrene,polyethylene, polypropylene or similar foam resin whose specific gravityis smaller than the specific gravity of the liquid 7. In theillustrative embodiment, use is made of polystyrene having specificgravity of 0.02 g/cm³ to 0.6 g/cm³. As for configuration, the floats 140should preferably have as small a surface area as possible so as to bedesirably floatable. As the area of each float 140 at the side counterto the flow of the liquid 7 increases, the float 140 more vibrates andcauses turbulence to occur in the liquid 7. Further, as the area of eachfloat 140 increases, resistance to floating increases in the liquid 7and thereby degrades the response of the float 140.

[0094] In the liquid level sensor 135 shown in FIG. 4A, the tonercontained in the liquid 7 and deposited on the spherical floats 140drops on and along the spherical surfaces of the floats 140 togetherwith the liquid carrier. The toner then reaches the liquid level in thesecond reservoir 115 and joins the liquid 7. The toner thereforeaccumulates on the floats 140 little. This reduces the sinking of thefloats 140 ascribable to the toner deposited thereon and thereby reducesa difference between the actual liquid level in the reservoir 115 andthe liquid level sensed by the sensor 135. Further, the spherical floats140 are subjected to less resistance ascribable to the flow of theliquid 7 than rectangular floats and therefore move little in the up anddown direction.

[0095] Moreover, the liquid level sensor 145 allows a minimum of changeto occur in the level of the magnetic force generating member 144 due tothe waving of the liquid level and thereby reduces the differencebetween the actual liquid level and the sensed liquid level. This willbe described specifically with reference to FIGS. 5A and 5B. Assume thatthe liquid 7 in a still condition shown in FIG. SA is agitated to wave.Then, as shown in FIG. 5B, the floats 140 each float at a particularliquid level, causing the ring pin 141 to incline. As a result, themagnetic force generating member 144 is positioned at a levelsubstantially intermediate between the above liquid levels.

[0096] In FIG. 5B, assume that a liquid level X1 where the right float140 is positioned is the original liquid level in the still condition.Then, the left float 140 is floating at a level lower than the originallevel X1 by a distance L. Should only one float be used, the magneticforce generating member 144 would also sink by the distance L. In theillustrative embodiment, the magnetic force generating member 144 sinksonly by one-half of the distance L, i.e., L/2.

[0097] Conversely, assume that the liquid level X2 where the left float140 is positioned is the original liquid level. Then, the right float140 rises above the original liquid level X2 by the distance L. However,the magnetic force generating member 144 rises only by a distance ofL/2.

[0098] Further, assume that one float 140 falls below an original liquidlevel while the other float 140 rises above the original liquid level.Then, the magnetic force generating member 144 can float at a levelclose to the original liquid level.

[0099] As stated above, the liquid level sensor 135 causes the level ofthe magnetic force generating member 144 and that of the magnetic forcesensing means to vary little despite waving. This successfully reducesthe difference between the actual liquid level in the reservoir and theliquid level sensed by the sensor 135.

[0100] The ring pin 141 inclines when the two floats 140 each float at aparticular liquid level, as stated earlier. The maximum inclination ofthe ring pin 141 can be delicately set only if the distance between thetwo ring portions 141 a and the clearance between the inner wall of eachring portion 141 a and the associated guide rod 139 are adjusted.

[0101] In the illustrative embodiment, the kind of the liquid 7 that theuser of the printer is expected to use is specified. Specifically, anoperation manual attached to the printer may include a messagespecifying the kind of the liquid 7, e.g., “Use a liquid A availablefrom a company B.” The floats 140 are formed of foam resin smaller inspecific gravity than the liquid 7 of the specified kind. Therefore, solong as the user uses the specific liquid 7, the floats 140 do not sinkin the liquid 7 at all.

[0102] The output signals of the Hall elements 143 are sent to thereplenishment controller to be described later. The replenishmentcontroller determines whether or not at least one of the Hall elements143 is outputting a sense signal. If the result of this decision isnegative, then the replenishment controller displays an error messageon, e.g., a display not shown. If the answer of the above decision ispositive, the replenishment controller determines whether or not thesecond Hall element 143 from the bottom responsive to the lower limit isoutputting a sense signal. If the second Hall element 143 is outputtinga sense signal, then the replenishment controller determines that theliquid 7 has decreased to the lower-limit liquid level or below.

[0103] In the illustrative embodiment, the Hall elements 143 are soarranged as to obviate a dead space as to sensing. The liquid levelsensor 135 is configured to sense the variation of the liquid level over20 mm.

[0104] As shown in FIG. 4B, the liquid level sensor 135 is constructedinto a unit together with the content signal outputting device 118. Sucha unit can be arranged in the second reservoir 115 in a compactconfiguration.

[0105]FIGS. 6A and 6B show a comparative configuration of the liquidlevel sensor 135. As shown, the sensor 135 includes a single float 140implemented as a square block. In this configuration, toner is likely toaccumulate on the top of the float 140 and causes the float 140 to floatat a level lower than the expected level due to the weight of the toner.Further, the float 140 cannot cause the ring pin 141 to incline, asshown in FIG. 5B, failing to reduce the variation of the level of themagnetic force generating member 144 ascribable to waving.

[0106]FIG. 7 shows essential part of electric circuitry arranged in theprinter. As shown, a control unit or control means 200 includes asensing controller 20, a replenishment controller 202, and a memory 203.The sensing controller 201 and replenishment controller 202 areinterconnected to interchange data with each other and are connected tothe memory 203. Further, the sensing controller 201 is connected to thedisk motor 133 assigned to the disk unit and photosensor 132. Thereplenishment controller 202 is connected to the conveyance pump 120,liquid pump 146, carrier pump 147 and liquid level sensor 135 as well asto an agitation motor 148 assigned to the paddles 116 and 117.

[0107] Reference will be made to FIG. 8 for describing a specificprocedure to be executed by the sensing controller 201. The sensingcontroller 201 executes the procedure to be described at a preselectedperiod. As shown, the sensing controller 201 starts driving the diskmotor 133 to thereby rotate the disk unit included in the content signaloutputting device (step S1). As a result, the liquid 7 in the secondreservoir 115 forms a film on the disk unit, as stated earlier.

[0108] Subsequently, the sensing controller 201 integrates thecontinuous output of the light-sensitive element of the photosensor 132over a preselected period of time (step S2). The preselected period oftime is one necessary for the disk unit to complete one rotation and isselected to be about 7 seconds in the illustrative embodiment. Oncompleting the step S2, the sensing controller 201 calculates the tonercontent of the liquid in the second reservoir 115 on the basis of theresult of integration (step S3). For this purpose, the sensingcontroller 201 may scan a database, which lists correspondence betweenintegrated values and toner contents, in order to find a toner contentmatching with the result of integration. Alternatively, the sensingcontroller 201 may substitute the result of integration for an algorithmrepresentative of a relation between the integrated value and the tonercontent.

[0109] After the step S3, the sensing controller 201 updates tonercontent data stored in the memory 203 with the calculated toner content(S4) and then determines whether or not to continue the controlprocedure (step S5). If the procedure should be continued because, e.g.,development is under way (Y, step S5), then the sensing controller 201returns to the step S2. If the answer of the step S5 is negative (N),then the sensing controller 201 stops driving the disk motor 133 (stepS6) and ends the procedure.

[0110] The procedure described above repeats integration until thesensing controller 201 confirms the stop of the disk motor 133 in thestep 5. As a result, the toner content data stored in the memory 203 isrepeatedly updated. While the step S2 consumes about 7 seconds, theother processing completes almost instantaneously. Therefore, a singletoner content calculating time (steps S2 through S5) required is only alittle over 7 seconds.

[0111]FIG. 9 shows a specific procedure to be executed by thereplenishment controller 202. Briefly, the replenishment controller 202replenishes either one of the carrier liquid and developing liquid tothe second reservoir 115 in accordance with the level of the liquid 7 inthe reservoir 115 and the toner content data stored in the memory 203.As a result, the toner content of the liquid 7 is controlled to thestandard toner content mentioned earlier.

[0112] The procedure of FIG. 9 will be outlined first. The replenishmentcontroller 202 determines the liquid level in the second reservoir 115(step S1) and then determines whether or not the liquid level is lowerthan the standard liquid level (level B) (step S2). If the answer of thestep S2 is Y, then the replenishment controller 202 replenishes thecarrier liquid or the developing liquid to the reservoir 115 over thepreselected period of time (step S11 or S14) and again executes the stepS1. If the actual liquid level is higher than the standard liquid level(N step S2), then the replenishment controller 202 ends the procedurewithout any replenishment. In this manner, when the replenishmentcontroller 202 started the procedure at a preselected period determinesthat the actual liquid level is lower than standard level, itreplenishes the carrier liquid or the developing liquid until the formerrises to the latter.

[0113] If the actual liquid level is coincident with the standard liquidlevel, then the step S1 is followed by steps S2, S3, S4 and S17. In thestep S17, the replenishment controller 202 determines whether or not tocontinue the procedure. If the procedure should be continued because,e.g., development is under way (Y, step S17), then the controller 202returns to the step S1. If the answer of the step S17 is N, thecontroller 202 ends the procedure.

[0114] So long as the printer is free from errors, the replenishmentcontroller 202 executing the above procedure maintains the actual liquidlevel around the standard level because the actual level varies betweenthe standard level and a level slightly below the standard level.However, there is a fear that the actual liquid level falls below thelower limit (level A) or rises above the upper limit (level C) due to,e.g., the failure of any one of the pumps and liquid level sensor 135 orthe consumption of the carrier liquid or the developing liquid to bereplenished. To solve this problem, the replenishment controller 202determines whether or not the actual liquid level is abnormal (step S4or S5). If the liquid level is abnormal (Y, step S4 or S5), then thecontroller 202 displays a liquid level error on the display and ends theprocedure.

[0115] Further, the replenishment controller 202 determines whether ornot the control agent has been used up, i.e., whether or not the liquidbottle 124 or the carrier bottle 123 has run out of the developingliquid or the carrier liquid. If the control agent has been used up,then the replenishment controller 202 displays a bottle error message,e.g., “Liquid bottle is empty.” or “Carrier bottle is empty.” on thedisplay and ends the procedure. More specifically, the controller 202drives the liquid pump 146 or the carrier pump 147 over a preselectedperiod of time to thereby replenish the developing liquid or the carrierliquid as a control agent (step S11 or S15). If the liquid level doesnot rise despite repeated replenishment, then the controller 202determines that the liquid bottle 124 or the carrier bottle 123 isempty. In the illustrative embodiment, the controller 202 drives theliquid pump 146 or the carrier pump 147 for 2 seconds each time andagain determines the liquid level (step S1). If the liquid leveldetermined in the step S1 is short of the standard level (level B), thenthe controller 202 again drives the liquid pump 146 or the carrier pump147.

[0116] Every time the controller 202 drives the liquid pump 146 or thecarrier pump 147, it increments the drive count C1 of the pump 146 orthe drive count C2 of the pump 147 by 1 (one) (step S12 or S16). Thecontroller 202 then determines whether or not the drive count C1 or C2has reached “10” (step S9 or S13). If the answer of the step S9 or S13is Y, then the controller 202 displays the bottle error message (stepS10 or S14). When the liquid level reaches the standard level (level B),the controller 202 resets the drive count C1 or C2 to zero (step S3) andthen ends the procedure.

[0117] The controller 202 determines which of the developing liquid andcarrier liquid should be replenished by referencing data stored in thememory 203. Specifically, the controller 202 reads the previouslymentioned toner content data and a target toner content stored in thememory 203 (step S7) and compares them (step S7). If the toner contentdata is smaller than the target toner content (N, step S8), then thecontroller 202 replenishes the developing liquid. If the toner contentdata is greater than or equal to the target toner content (N, step S8),then the controller 202 replenishes the carrier liquid. In theillustrative embodiment, the target toner content is selected to be 18%.Therefore, the controller 202 replenishes the carrier liquid if thetoner content reaches 18%. While a toner content desirable fordevelopment, like the toner content of the liquid in the liquid bottle124, is 15%, development density does not vary if the above tonercontent is 15±3%, i.e., lies in a range of from 12% to 18%.

[0118] The toner content of the developing liquid stored in the liquidbottle 124 is 15%, which is the standard toner content. Therefore, whenthe toner content of the liquid 7 in the second reservoir 115 is above15%, but below 18%, the replenishment of the liquid from the liquidbottle 124 dilutes the liquid 7 in the reservoir 115. This, however,successfully reduces the fall of toner content ascribable toreplenishment in the range of from 15% to 18% and thereby reduces shortdevelopment density, compared to the case wherein the target density isselected to be 15%. The developing liquid to be replenished from theliquid bottle 124 may be provided with a toner content higher than 15%,if desired.

[0119] The pump drive effected in the step S11 or S15 consumes 2seconds, as stated above. However, the other processing completes almostinstantaneously, so that the steps S1 through S12 or the steps Sithrough 16 consumes only a little over 2 seconds. On the other hand, thetoner content data is not updated over a little over 7 seconds, which isa single toner content calculating time. It follows that when thecontroller 202 repeatedly controls the toner content, the same tonercontent data is read out of the memory 203 and used at least threetimes.

[0120] As stated above, the replenishment controller 202 can execute theexpected control based on the toner content data stored in the memory203 without waiting until the sensing controller 201 calculates a tonercontent.

[0121] In the illustrative embodiment, the replenishment controller 202uses the drive count C1 or C2 for determining whether or not the liquidbottle 124 or the carrier bottle 123 is empty. Alternatively, thebottles 123 and 124 each may be provided with sensing means responsiveto the amount of residual liquid and sending its output to thecontroller 202. The sensing controller 201 and replenishment controller202 may be implemented as, e.g., a single CPU (Central Processing Unit),so that control over toner content sensing and control over tonercontent adjustment can be executed in parallel.

[0122] When the actual liquid level reaches the emergency-stop liquidlevel, it is likely that the paddle 116 bodily appears above the liquidsurface and scatters the liquid 7. In this case, the replenishmentcontroller 202 stops the operation of the entire printer, although notshown in FIG. 9.

[0123]FIG. 10 shows the liquid adjusting section 103 with the lid 119closing the second reservoir 115. As shown, the second reservoir 115including the liquid level sensor 135 has a cross-section that is notcircular, but is generally oblong. It should be noted that the term“generally oblong” does not refer to a geometrical oblong, but refers tothe combination of two halves of a circle and a square or a rectangleintervening between them. Such a configuration resembles, e.g., a trackand field stadium.

[0124] A shaft 138 is journalled to a water-proof bearing, not shown,mounted on the bottom of the reservoir 115 at a position offset from thecenter (center of gravity) of the bottom to the left, as viewed in FIG.10. The paddle 116 is formed of a flexible material and affixed to theshaft 138. The paddle 117 is formed of a non-flexible material andaffixed to the shaft 138 below the paddle 116. The flexible paddle 116has a length preselected such that the paddle 116 contacts the innerperiphery of the reservoir 115 at any angular position. When the shaft138 is rotated, the paddle 116 rotates in the same direction as theshaft 138 while deforming to slide on the inner circumference of thereservoir 115.

[0125] The support plate 129 and outer disks 131 a and 131 b are locatedon the locus of the flexible paddle 116. As shown in FIG. 11, when thepaddle 116 in rotation contacts the above members, it further deforms inthe direction opposite to the direction of rotation and slides on themembers. Subsequently, as shown in FIG. 12, the paddle 116 reaches aposition where it can be released form the members. Thereafter, as shownin FIG. 13, the paddle 116 again contacts the inner periphery of thereservoir 115. The paddle 116 can therefore reduce the viscosity of theentire liquid 7 in contact therewith over the entire locus of rotationdespite the presence of the support plate 129 and outer disks 131 a and131 b. More specifically, despite that the liquid 7 is thixotropic andtherefore difficult to convect, the paddle 116 causes the liquid 7 todesirably swirl at the level of the paddle 116 and thereby desirablyagitates it in the horizontal direction.

[0126] As shown in FIG. 10, the non-flexible paddle 117 is implementedby four blades arranged in the form of a propeller. Each blade extendsradially outward from the shaft 138 while being twisted. When the paddle117 rotates to cause the liquid 7 to swirl, the liquid 7 flows downward,as viewed in FIG. 10, to the bottom of the reservoir 115 along the axisof the shaft 138. The liquid 7 then flows upward away from the bottom ofthe reservoir 115. The locus of rotation of the paddle 117 does notextend over the entire sectional area of the reservoir 115 at the heightof the paddle 117. However, part of the liquid 7 reduced in viscosity onthe locus of rotation and rebounded from the bottom of the reservoir 115can efficiently migrate to the zone outside of the locus. This allows aminimum of difference in viscosity to occur between the liquid 7 presenton the locus of rotation and the liquid 7 present in the above zone,thereby insuring efficient agitation.

[0127] The downward flow of the liquid 7 along the axis of the shaft 138shown in FIG. 10 is more preferable than an upward flow. The downwardflow causes the liquid surface to wave less than the upward flow.Moreover, the downward flow exerts a strong shearing force on the liquid7 because of the rebound from the bottom of the reservoir 115 to therebyreduce viscosity more than the upward flow.

[0128] In the illustrative embodiment, the non-flexible paddle 117exerts a greater agitating force than the flexible paddle 116 andtherefore reduces the viscosity of the liquid 17 at is level earlierthan the flexible paddle 116. The liquid 7 reduced in viscosity reboundsfrom the bottom of the reservoir 115 and rises to the level of thepaddle 116. In this sense, the paddle 117 helps the paddle 116 agitatethe liquid 7. Part of the liquid 7 around the center of the locus ofrotation of the paddle 116 is pulled downward to the level of the paddle117 by the axial flow generated by the paddle 117.

[0129] In the above configuration, the liquid 7 in the reservoir 115 ispositively rotated in the direction of rotation of the shaft 138 andsufficiently agitated in the horizontal direction. In addition, theliquid 7 flows downward around the axis of rotation and then flowsupward in the region remote from the axis and is therefore sufficientlyagitated in the up-and-down direction as well.

[0130] Assume that the flexible paddle 116 simply causes the liquid 7 toflow in contact therewith over the entire sectional area of thereservoir 115. Then, the sophisticated configuration described withreference to FIGS. 10 and 11 is not necessary. Specifically, as shown inFIG. 14, the paddle 116 will achieve the above function only if thereservoir 115 has a circular cross-section and if the paddle 116 rotatesat the center of the cross-section. In this simple configuration,however, the non-flexible paddle 117 exerting a greater agitating forcethan the paddle 116 also rotates at the center of the abovecross-section and causes the liquid surface to noticeably wave due tothe relatively high-speed rotation and axial flow of the liquid 7, asshown in FIG. 15. Such waving brings about a noticeable differencebetween the actual liquid level in a still condition and the liquidlevel sensed by the sensor 135. Further, an eddy around the shaft 138pulls the floats 140 and causes them to move up and down, aggravatingthe difference mentioned above.

[0131] In light of the above, in the illustrative embodiment, the shaft138 is located at a position offset from the center of the reservoir 115and causes the paddles 116 and 117 to rotate thereabout. The paddle 117causes the liquid 7 to positively swirl at the zone closer to the shaft138 than the center of the reservoir 115, causing the liquid surface towave. On the other hand, the liquid 7 at the side opposite to the shaft138 with respect to the center of the reservoir 115 relatively slowlyrotates because of the convection of the liquid 7 caused by the paddle117 or the rotation of the paddle 116, which exerts a weaker agitatingforce than the paddle 117. This part of the liquid 7 therefore causesthe liquid surface to wave little. The liquid level sensor 135 sensesthe liquid level at the position where the liquid surface waves little,and is therefore free from critical sensing errors ascribable to waving.

[0132] As shown in FIG. 16, the bottom of the reservoir 115 shouldpreferably be formed of a taper 145. The taper 145 is configured suchthat the liquid rebounded from the bottom of the reservoir 115 and thenspread along the bottom flows upward along the taper 145. In thisconfiguration, while the liquid 7 flows downward around the shaft 138,it positively flows upward at the side remote from the shaft 138. Theliquid 7 is therefore surely agitated in the up-and-down direction.

[0133] In the illustrative embodiment, the toner content sensing means,i.e., the content signal outputting device 118 and control unitdetermine the toner content of part of the liquid 7 guided by the taper145 upward. Further, as shown in FIG. 17, the control agent (developingliquid or carrier liquid for replenishment) and liquids collected fromthe drum 1 and developing roller 105, FIG. 1, drop to the liquid surfaceat positions closer to the shaft 138 than the center of the reservoir115, i.e., where the paddle 117 causes the liquid 7 to positivelyrotate. More specifically, such liquids drop to the liquid surface abovethe locus of rotation of the paddle 117.

[0134] The axial flow generated by the paddle 117 entrains the liquidsdropped to the liquid surface toward the bottom of the reservoir 115, sothat the liquids are sufficiently agitated in the horizontal andup-and-down directions. Further, on the bottom of the reservoir 115, theliquids are guided by the taper 145 upward while spreading from theshaft 138 side to the opposite side. Subsequently, the liquids arefurther agitated in the above directions in the liquid 7 existing at theside opposite to the shaft 138 because of the upward flow and therotation of the paddle 116. The toner content sensing means thereforesenses the toner content of the liquid 7 in which the control agent andcollected liquids are sufficiently agitated in the horizontal andup-and-down directions.

[0135]FIG. 18 shows a modified form of the paddle 116 of FIG. 10. Asshown, the paddle 116 is made up of a flexible, upper paddle 116 apositioned above the non-flexible paddle 117 and a flexible, lowerpaddle 116 b positioned below the paddle 117. The lower paddle 116 bagitates toner precipitated on the bottom of the reservoir 115 due to,e.g., a long time of suspension, thereby further promoting thedispersion of the toner in the liquid 7.

[0136] Metal powder is sometimes introduced in the liquid 7 in the firstreservoir 110 and/or the second reservoir 115. Such metal powder isderived from friction acting between rollers and blades formed of metal,e.g., collecting roller 111, blade 112, coating roller 106 and meteringblade 107 as well as meshing between gears not shown. The liquid 7containing metal powder is apt to cause a bias to leak at the developingposition or the nip for image transfer, deteriorating the developingability and image transferring ability. Further, the liquid 7 with metalpowder is apt to cause discharge to occur from the developing roller 105to the drum 1; the discharge would damage the surface of the drum 1. Inaddition, the liquid 7 with metal powder is likely to scratch the disksof the disk unit and to vary the transmittance of the liquid film.These, of course, result in low image quality.

[0137] Considering the above situation, it is preferable to disposeremovable magnets in the reservoirs 110 and 115 or the pipes forconveying the liquid so as to collect metal powder. Alternatively, amagnet may be located between the position where the coating roller 106and developing roller 105 contact and the developing position and spacedby a preselected distance from the roller 105. More preferably, thismagnet should be implemented as a magnet roller spaced from thedeveloping roller 105 by a gap of 10 μm to 200 μm, in which case a bladewill be used to scrape off metal powder deposited on the magnet roller.In this case, the magnet roller can magnetically collect metal powderfrom the liquid 7 immediately before development.

[0138] While the illustrative embodiment has concentrated on a printerof the type forming a monochromatic image, it is similarly applicable toany other type of image forming apparatus. For example, there isavailable an image forming apparatus of the type including a yellow, amagenta, a cyan and a black developing unit arranged around aphotoconductive element or similar image carrier. The developing unitseach develop a latent image formed on the image carrier in a particularcolor. The resulting toner images of different colors are transferred toan intermediate image transfer body one above the other, completing afull-color image. In another type of image forming apparatus, imagecarriers each being assigned to a particular color are arranged inaddition to such developing units. In this type of apparatus, tonerimages of different colors are transferred to a paper sheet or similarrecording medium one above the other without the intermediary of theintermediate image transfer body. Still another type of image formingapparatus writes a latent image on an image support body by anionographic method.

[0139] As stated above, the illustrative embodiment has variousunprecedented advantages, as enumerated below.

[0140] (1) The illustrative embodiment causes the floats to sink littledue to the deposition of toner or causes the level of the magnetic forcegenerating member or that of the magnetic force sensing means to varylittle. This reduces a difference between the actual liquid level in thereservoir and the liquid level sensed by the liquid level sensor.

[0141] (2) Wires included in the magnetic force sensing means arepreventing from twining round due to the up-and-down movement of thefloats.

[0142] (3) A plurality of different liquid levels can be sensed on thebasis of the up-and-down movement of a single magnetic force generatingmember.

[0143] (4) The floats are preventing from loosing floatability due todamage to the outer walls of the floats.

[0144] (5) Toner in the reservoir can have a uniform content. The liquidlevel sensor is free from critical errors ascribable to the waving ofthe developing liquid in the reservoir.

[0145] (6) The developing liquid can be sufficiently agitated in thehorizontal and up-and-down directions without waving ascribable toagitation. This also enhances the uniformity of the toner content.

[0146] (7) The rise and waving of the liquid surface are reduced,compared to the case wherein the liquid flows upward to the liquid leveland then flows downward.

[0147] (8) The liquid is smoothly agitated in the up-and-down directionand can therefore have its toner content more surely uniformed.

[0148] (9) Even when the toner content of the liquid after developmentdiffers from the toner content before development, the liquid can bereused for development with the toner content adjusted to one beforedevelopment. Further, the toner content of the liquid can be adjustedwhile maintaining an adequate liquid level in the reservoir.

[0149] (10) The control means can determine whether or not toner ispresent in the replenishing means without resorting to an exclusivesensor.

[0150] (11) The liquid has a toner content as high as 5% to 40% and cantherefore form a high density image in a smaller amount than a liquidhaving a lower toner content. This kind of liquid is low cost than aliquid whose viscosity is 10,000 mPa·s or above. In addition, such aliquid reduces irregularity in image density ascribable to the irregulardispersion of toner, compared to a liquid having viscosity less than 100mPa·s that is likely to bring about irregular dispersion.

[0151] Second Embodiment

[0152] This embodiment is directed mainly toward the second and thirdobjects stated earlier. FIGS. 1, 4A, 4B, 6A, 6B and 7 through 18referenced in relation to the first embodiment and description thereofalso apply to the second embodiment. To better understand the secondembodiment, reference will be made to a conventional developing unit,shown in FIGS. 19 and 20.

[0153] As shown in FIG. 19, the developing unit includes a hollowcylindrical reservoir 150 storing a thixotropic developing liquid notshown. A shaft 138 is positioned in the reservoir 150 around the axis Xof the reservoir 150. Drive means, not shown, causes the shaft 138 todrive. A paddle or agitating member 116 is affixed to thecircumferential surface of the shaft 138. The paddle 116 has such alength that it rotates with a radius slightly smaller than the insidediameter of the reservoir 150. When the shaft 138 and therefore thepaddle 116 starts rotating, the paddle 116 reduces the viscosity of theliquid directly contacting the paddle 116 on the locus of rotationlittle by little and causes the liquid to swirl about the axis X. Therotation of the paddle 116 is transferred even to the liquid remote fromthe paddle 116 soon in the up-and-down direction of the reservoir 150.As a result, the entire liquid in the reservoir 150 swirls about theaxis X and decreases in viscosity little by little. At the same time,toner or similar developing substance is evenly dispersed over theentire liquid, providing the liquid with a uniform toner content.

[0154] A liquid level sensor is sometimes disposed in the reservoir 150for sensing the liquid level of the liquid, as stated earlier. Inpractice, as shown in FIG. 20, an eddy formed at the center zone lowersthe liquid level below the original liquid level WL while the liquidlevel in the peripheral zone rises above the original level WL. Further,while the liquid level is shown in FIG. 20 as being symmetrical in theright-and-left direction, the liquid surface, in practice, noticeablywaves in a complicated configuration. This waving is ascribable to,e.g., friction acting between the liquid and the wall of the reservoir150 and exerting a force that obstructs the swirl of the liquid.Consequently, the liquid level sensed by the liquid level sensornoticeably differs from the actual liquid level.

[0155] On the other hand, a sensor responsive to the toner content ofthe liquid is sometimes disposed in the reservoir 150. The sensor mustbe positioned at the same level as the paddle 116 for layout reasons, asthe case may be. In such a case, as shown in FIG. 23, the shaft 138 maybe located at a position offset from the axis X of the reservoir 150while the paddle 116 may be reduced in length, so that the paddle 116does not contact the sensor labeled 151. This, however, prevents thepaddle 116 from agitating the entire liquid at the level of the paddle116 and therefore obstructs efficient agitation.

[0156] Hereinafter will be described part of the second embodimentdifferent from the first embodiment. As shown in FIGS. 21 and 22, theillustrative embodiment includes a liquid level sensor 135 constructedto optically sense the liquid level of the developing liquid in thecylindrical, second reservoir 115, which is formed of an opaquematerial. Specifically, the liquid level sensor 135 includes a pair ofglass blocks 125 and 126. The glass block 125 supports threelight-sensitive elements 127 a through 127 c while the glass block 126supports three light emitting elements 128 a through 128 c aligning withthe light-sensitive elements 127 a through 127 c, respectively.

[0157] More specifically, as shown in FIG. 21, the glass blocks 125 and126 are spaced from each other by a preselected distance and affixed tothe second reservoir 115 in such a manner as to pierce thecircumferential wall of the reservoir 115. The light emitting elements128 a through 128 c each are mounted on the outer periphery of one glassblock 126 at a particular level or height so as to emit light toward theother glass block 125 via the glass block 126 and air inside thereservoir 115. The light-sensitive elements 127 a through 127 c aremounted on the outer periphery of the glass block 125, and each receivesthe light issuing from aligned one of the light emitting elements 128 athrough 128 c via the glass block 125. The light emitting elements 128 athrough 128 c and light-sensitive elements 127 through 127 c provided inpairs are responsive to the lower-limit liquid level (level A), standardliquid level (level B) and upper-limit liquid level (level C),respectively. Assume that the liquid 7 does not exist on the opticalpath between any one of the light emitting elements 128 a through 128 cand associated one of the light-sensitive elements 127 a through 127 c.Then, light issuing from the light emitting element is smoothlytransmitted through the glass block 126, air inside the reservoir 115and glass block 125 to the light-sensitive element. However, when theliquid 7 exists on the above optical path, it interrupts part of or theentire light and thereby noticeably reduces the quantity of light toreach the light-sensitive element. The light-sensitive elements 127 athrough 127 c each send an analog signal representative of the quantityof incident light to the control unit not shown. The control unitconverts the analog signals received from the light-sensitive elements127 a through 127 c to digital signals and then determines thequantities of incident light at the levels A, B and C. The control unitthen determines the liquid level of the liquid 7 in the reservoir 115.

[0158] The combination of each of the light emitting elements 128 athrough 128 c and associated one of the light-sensitive elements 127 athrough 127 c may be implemented by, e.g., a transmission typephotoelectric switch. A photoelectric switch includes an LED (LightEmitting Diode) or similar light emitting element and a photodiode orsimilar light-sensitive element and outputs an ON signal when light isintercepted.

[0159] The inner periphery of the reservoir 115, including the walls ofthe glass blocks 125 and 126, are treated to repel oil for therebypreventing the liquid 7 from depositing thereon. The liquid 7 depositedon the walls of the glass blocks 125 and 126 would effect the quantityof light to be incident to the light-sensitive elements 127 a through127 c and would thereby prevent the control unit from accuratelydetermining a liquid level.

[0160] As stated above, the illustrative embodiment has variousunprecedented advantages, as enumerated below.

[0161] (1) The liquid level sensor is free from critical errorsascribable to the waving of the developing liquid in the reservoir.

[0162] (2) The developing liquid can be sufficiently agitated in thehorizontal and up-and-down directions without waving ascribable toagitation. This enhances the uniformity of the substance content.

[0163] (3) The rise and waving of the liquid surface are reduced,compared to the case wherein the liquid flows upward to the liquid leveland then flows downward.

[0164] (4) The liquid is smoothly agitated in the up-and-down directionand can therefore have its toner content more surely uniformed.

[0165] (5) Even when the toner content of the liquid after developmentdiffers from the toner content before development, the liquid can bereused for development with the toner content adjusted to one beforedevelopment. Further, the toner content of the liquid can be adjustedwhile maintaining an adequate liquid level in the reservoir.

[0166] (6) Even when a toner content sensor or similar member isdisposed in the reservoir at the same level at the agitating member, theagitating member is capable of causing the entire liquid to swirl incontact with the liquid while reducing its viscosity.

[0167] (7) The non-flexible agitating member helps the flexibleagitating member agitate the liquid without deforming itself, so thatthe liquid is more surely agitated.

[0168] (8) A driveline for causing the flexible and non-flexibleagitating members to rotate is simple, compared to a case wherein suchagitating members each are driven by a respective shaft. Thissuccessfully reduces the cost of the driveline.

Third Embodiment

[0169] This embodiment is directed mainly toward the fourth objectstated earlier. FIGS. 1, 4A, 4B, 6A, 6B and 7 through 19, 21 and 22referenced in relation to the first and second embodiments anddescription thereof also apply to the third embodiment.

[0170] As shown in FIG. 24, in the illustrative embodiment, thenon-flexible paddle 117 is rotatable with its front face, which contactsthe developing liquid, inclined relative to the axis of the shaft 138(vertical direction as viewed in FIG. 24). With this configuration, thepaddle 117 forces the liquid to rotate in the direction of rotation ofthe paddle 117 while forcing it to flow in the axial direction of theshaft 138 (downward as viewed in FIG. 24). At the same time, the rearface of the paddle 117 moves away from the liquid and thereby generatesnegative pressure between it and the liquid.

[0171] Assume that a single paddle 117 is present in the axial directionof the shaft 138. Then, due to the negative pressure generated by therear face of the paddle 117, an upper eddy A and a lower eddy B appearat the rear of the paddle 117 in the direction of rotation of the paddle117. The upper eddy A swirls such that is leaves the front end portionof the paddle 117 and again approaches it at the intermediate portion ofthe paddle 117. On the other hand, the lower eddy A swirls such that itleaves the rear end portion of the paddle 117 and again approaches it inthe intermediate portion of the paddle 117. The eddies A and B obstructthe rotation and axial movement of the liquid at the rear of the paddle117.

[0172] In light of the above, as shown in FIG. 25, a plurality of (twoin the illustrative embodiment) paddles 117 a and 117 b face each otherat a preselected distance in the axial direction of the shaft 138. Inthis configuration, the front face of the upper paddle 117 a forces theliquid to flow toward the rear face of the lower paddle 117 b,preventing the two eddies A and B, FIG. 24, from appearing at the rearof the lower paddle 117 b. The liquid can therefore move in the axialdirection of the shaft 138 while swirling in the direction of rotationof the paddles 117 a and 117 b.

[0173]FIGS. 26 and 27 show a modification of the paddle 117 of FIG. 25.As shown, a plurality of pairs (four pairs in the illustrativeembodiment) of upper and lower paddles 117 a and 117 b extend radiallyoutward from the shaft 138. The upper and lower paddles 117 a and 117 bof each pair face each other at a preselected distance while beinginclined as in FIG. 25. In the illustrative embodiment, the rear portionof each upper paddle 117 a is bent toward the lower paddle 117 b, i.e.,inclined more than the front portion relative to the axis of the shaft138. Further, the front end portion and rear end portion of each upperpaddle 117 a are positioned ahead of the front end portion and rear endportion of the lower paddle 117 b in the direction of rotation.

[0174] As shown in FIG. 27, the lower paddle 117 b forces the liquid toswirl in the direction of rotation of the paddle 117 with its front facewhile causing it to flow axially downward, as viewed in FIG. 27. Theupper paddle 117 a forces the liquid to swirl toward the rear face ofthe lower paddle 117 b with its front face, as indicated by an arrow C.The liquid therefore flows downward while swirling without encounteringany eddy at the rear of the paddle 117 b. At this instant, the bent,rear portion of the upper paddle 117 a forces the liquid toward the rearsurface of the lower paddle 117 b more strongly than the front portionof the paddle 117 a. This successfully causes the liquid to more surelymove downward while swirling in the direction of rotation of the paddle117 a, thereby more positively obviating the eddies at the rear of thepaddle 117 b.

[0175] Furthermore, the front end portion of the upper paddle 117 a ispositioned ahead of the front end portion of the lower paddle 117 b inthe direction of rotation, guiding the liquid to the space between theupper paddle 117 a and the lower paddle 117 b. This part of the liquidsmoothly flows through the above space and then drags the liquidcontacting the rear face of the upper paddle 117 a, causing it to flowin a direction E. The drag obviates eddies otherwise appearing at therear of the upper paddle 117 a and thereby further enhances theefficient swirl and downward movement of the liquid.

[0176] As stated above, the illustrative embodiment has variousunprecedented advantages, as enumerated below.

[0177] (1) The paddles sufficiently agitate the developing liquid inboth of the horizontal and up-and-down directions while preventing theliquid surface from rising or waving, as in the previous embodiments.

[0178] (2) The liquid swirls in the direction of rotation of the paddlesand moves in the axial direction without encountering any eddy at therear of the lower paddle. The paddles therefore agitate the liquid moreefficiently.

[0179] (3) The liquid swirls more surely in the direction of rotation ofthe paddles at the rear of the lower paddle, while moving in the axialdirection of the shaft. This further promotes the agitation of theliquid in the horizontal and up-and-down directions.

[0180] (5) The liquid is efficiently agitated in the direction ofrotation of the shaft even at the rear of the upper paddle, while movingin the axial direction of the shaft. This also further promotes theagitation of the liquid in two directions.

Fourth Embodiment

[0181] This embodiment is directed mainly toward the fifth object statedearlier. FIGS. 3, 4A, 4B and 7 through 15 referenced in relation to theprevious embodiments and description thereof also apply to the fourthembodiment. Further, as shown in FIG. 28, the printer of the fourthembodiment is identical with the printer of FIG. 1 except that aresidual liquid reservoir 160 and a collection pump 161 intervenebetween the drum cleaner 4 and the second reservoir 115. In the figures,identical structural elements are designated by identical referencenumerals and will not be described in order to avoid redundancy.

[0182] In FIG. 28, part of the toner facing the background or non-imagearea of the drum 1 does not move toward the drum 1, but gathers towardthe surface of the developing roller 105 by electrophoresis.Theoretically, therefore, such toner is not expected to deposit on thebackground of the drum 1. In practice, however, toner with short charge,for example, is apt to deposit on the background by moving later thanthe other toner by electrophoresis, bringing about so-called fog orbackground contamination.

[0183]FIG. 29 shows a sweeping unit 170 usable to obviate theabove-mentioned fog. As shown, the sweeping unit 170 includes a sweeproller 171, a cleaning blade 172, and a screw 173 for collection. Thecircumference of the sweep roller 171 is covered with a conductive,elastic material, e.g., conductive urethane rubber. The sweep roller 171rotates at the same speed as the drum 1 in contact with the drum 1,thereby forming a nip. A power source, not shown, applies a bias of thesame polarity as the toner to the sweep roller 171. As a result, anelectric field is formed between the drum 1 and the sweep roller 171 dueto a potential difference between the drum 1 and the roller 171. Morespecifically, at the above nip, the sweep roller 171 and the backgroundand latent image of the drum 1 are of the same polarity as the toner;the potential sequentially decreases from the background to the latentimage via the sweep roller 171. In this condition, the toner failed togather on the surface of the developing roller 105 moves toward thesweep roller 171 at the position between the background and the roller171 by electrophoresis. The sweep roller 171 therefore removes the tonerotherwise bringing about fog.

[0184] The cleaning blade 172 collects the liquid, which contains thetoner, removed by the sweep roller 171. The liquid is then returned tothe second reservoir 115 via the collecting section 102.

[0185] In the illustrative embodiment, the carrier bottle 123 formingpart of the replenishing section 104 stores a carrier liquid, or controlagent, for diluting the liquid 7 in the second reservoir 115. Thecarrier pump 147 delivers the carrier liquid from the carrier bottle 123to the second reservoir 115 under the control of the control unit notshown. The liquid bottle 124 stores a developing liquid, or anothercontrol agent, for thickening the liquid 7 in the second reservoir 115.The liquid pump 146 delivers the adjusting liquid to the secondreservoir 115 under the control of the control unit.

[0186] While the preferable or standard toner content of the liquid 7 inthe illustrative embodiment is also 15 wt %, irregular image densityobservable by eye does not occur if the toner content is 12 wt % to 18wt %. Therefore, in the illustrative embodiment, toner contents lying inthe range of from 12 wt % to 18 wt % are allowable.

[0187] In the illustrative embodiment, the control agent stored in theliquid bottle 124 has a toner content higher than the standard tonercontent (15 wt %).

[0188] The control unit compares the actual toner content represented bythe output of the content signal outputting device 118 and a preselectedtarget value. The control unit controls, based on the result ofcomparison, the drive of the pump 146 or 147 for thereby replenishing anadequate amount of carrier liquid or an adequate amount of adjustingliquid to the second reservoir 115. The liquid 7 in the second reservoir115 can therefore confine its toner content in the allowable rangedespite the return of the residual liquid collected from the developingroller 105 and the residual liquid collected from the drum 1. Let theresidual liquid collected from the developing roller 105 and theresidual liquid collected from the drum 1 be referred to as a residualdevelopment liquid and a residual image transfer liquid, respectively.

[0189] Arrangements unique to the illustrative embodiment will bedescribed hereinafter. The carrier liquid has a toner content of 0 wt %far different from the standard toner content. The carrier liquidtherefore rapidly lowers the toner content of the liquid 7 in the secondreservoir 115 when replenished to the reservoir 115.

[0190] On the other hand, in a printer in which the liquid 7 isthixotropic, as in the illustrative embodiment, the control agent storedin the liquid bottle 124 is required to have a toner content close tothe standard toner content (15 wt %). This is because the control agentcannot exhibit the properties of a liquid unless the toner contentthereof is limited. It follows that the control agent must bereplenished in a far greater amount than the traditional low tonercontent, low viscosity developing liquid. However, the amount of controlagent that can be replenished to the second reservoir 115 is limited.Even if the control agent is replenished to the upper limit, then it mayfail to effectively increase the toner content of the liquid 7 in thereservoir 115. This is likely to cause the toner content of the liquid 7in the reservoir 115 to sequentially decrease below the lower limit,i.e., 12 wt %.

[0191] At the intermediate transfer position where the drum 1 contactsthe intermediate drum 3, a toner image is electrostatically transferredfrom the drum 3 to the intermediate drum 3. As a result, the residualliquid left on the drum 1 after the image transfer contains only a smallamount of toner. This liquid therefore lowers the toner content of theliquid 7 when returned to the second reservoir 115. If the toner contentof the liquid 7 in the reservoir 115 decreases due to the residual imagetransfer liquid at a higher rate than it increases due to thereplenished agent, then the toner content of the liquid 7 continuouslydecreases below the lower limit (12 wt %)

[0192] To solve the problem discussed above, the illustrative embodimentdoes not directly return the residual image transfer liquid to thesecond reservoir 115, but once collects it in the residual liquidreservoir 160. The residual image transfer liquid collected in thereservoir 160 is used as a control agent for diluting the liquid 7 inthe second reservoir 115. More specifically, assume that the tonercontent represented by the output of the content signal outputting means118 is higher than the target toner content. Then, the drive unit drivesthe collection pump 161 for delivering the residual image transferliquid from the residual liquid reservoir 160 to the second reservoir115. Assume that the delivery of the liquid from the reservoir 160 failsto lower the actual toner content to the target toner content in apreselected period of time or that the liquid level in the reservoir 115does not rise to the preselected level in the preselected period oftime. Then, the control unit determines that the reservoir 160 has runout of the residual image transfer liquid, and drives the carrier pump147 to replenish the carrier liquid from the carrier bottle 123.

[0193] The procedure described above successfully stabilizes the tonercontent of the liquid 7 in the reservoir 115 and confines it in theallowable range of from 12 wt % to 18 wt %, compared to the conventionalprocedure that unconditionally returns the residual image transferliquid.

[0194]FIG. 30 demonstrates a specific toner content control procedure tobe executed by the replenishment controller (simply controllerhereinafter) 202, FIG. 7. As shown, the controller 202 determines theactual liquid level in the second reservoir 115 (step S1). If the actualliquid level is lower than the standard liquid level (Y, step S2), thenthe controller 202 replenishes one of the residual image transferliquid, carrier liquid and developing liquid for replenishment to thereservoir 115 for a preselected period of time (step S9, S11 or S13) andagain determines the actual liquid level (step S1). If the actual liquidlevel is higher than or equal to the standard liquid level (N, step S2),then the controller 202 ends the procedure without any replenishment.The controller 202 executes the above control at a preselected period.

[0195] More specifically, if the actual toner content is lower than thetarget toner content (Y, step S8), the controller 202 drives the liquidpump 146 for 2 seconds and then returns to the step S1. The liquid pump146 delivers a preselected amount of developing liquid to the reservoir115 to thereby slightly increase the toner content of the liquid 7.

[0196] If the actual toner content is higher than the target tonercontent (N, step S8), then the controller 202 drives the collection pump161 for 2 seconds and then returns to the step S1. Usually, thecollection pump 161 delivers a preselected amount of residual imagetransfer liquid from the residual liquid reservoir 160 to the secondreservoir 115 to thereby lower the toner content of the liquid 7.However, the residual image transfer liquid is not always present in theresidual liquid reservoir 160, i.e., it is not always replenisheddespite the drive of the collection pump 161. The controller 202therefore increments a drive count or variable C1 by 1 every time itdrives the collection pump 161 for 2 seconds (step S12). When the countC1 reaches “10”. The controller 202 stops driving the collection pumpand starts driving the carrier pump 147 for 2 seconds (step S13). Thecarrier pump 147 replenishes the carrier liquid to the reservoir 115 tothereby dilute the liquid 7.

[0197] By the above procedure, the controller 202 controls the actualtoner content of the liquid 7 in the reservoir 115, whether it be lowerthan or higher than the target content, to the target content.Therefore, although the toner content of the residual image transferliquid returned to the reservoir 115 may increase or decrease, the tonercontent of the liquid 7 in the reservoir 115 can be adequatelycontrolled.

[0198] Further, the residual image transfer liquid is returned from theresidual liquid reservoir 160 to the second reservoir 115 only when theliquid 7 in the reservoir 115 should be diluted. This successfullystabilizes the toner content of the liquid 7 and confines in theallowable range of from 12 wt % to 18 wt %, compared to the conventionalprocedure that unconditionally returns the residual image transferliquid.

[0199] Moreover, when the amount of residual image transfer liquid inthe residual liquid reservoir 160 is short, the carrier liquid isreplenished to the second reservoir 115 in place of the residual imagetransfer liquid, adequately lowering the toner content of the liquid 7.

[0200] In the illustrative embodiment, the target toner content used inthe step S8 is higher than the standard toner content (15 wt %), butlower than or equal to the upper limit (18 wt %) of the allowable range.It follows that even when the toner content of the liquid 7 in thereservoir 115 increase slightly above the standard toner content, thedeveloping liquid for replenishment is continuously replenished.Subsequently, when the toner content of the liquid 7 further increases,the residual image transfer liquid or the carrier liquid is replenishedin place of the above liquid. This allows the toner content to belowered at a later timing than when the residual image transfer liquidor the carrier liquid is substituted for the developing liquid as soonas the toner content exceeds 15 wt %. Consequently, a sharp decrease inthe toner content of the liquid 7 and therefore short image densityascribable to short toner content is obviated.

[0201] If the actual liquid level is equal to the standard liquid level,as determined in the step S2, then the controller 202 sequentiallyexecutes steps S3, S4 and S14. In the step S14, the controller 202determines whether or not the control should be executed. If the answerof the step S14 is Y, meaning that the control should be continuedbecause development, for example, is under way, then the controller 202returns to the step S1; if otherwise, the controller 202 ends theprocedure.

[0202] So long as the printer is free from errors, the controller 202executing the above procedure maintains the actual liquid level aroundthe standard level because the actual level varies between the standardlevel and a level slightly below the standard level. However, there is afear that the actual liquid level falls below the lower limit or risesabove the upper limit due to, e.g., the failure of any one of the pumpsand liquid level sensor 135 or the consumption of the carrier liquid orthe developing liquid to be replenished. To solve this problem, thecontroller 202 determines whether or not the actual liquid level isabnormal (step S4 or S5). If the liquid level is abnormal (Y, step S4 orS5), then the controller 202 displays a liquid level error on thedisplay and ends the procedure.

[0203] The pump drive effected in the step S9, S11 or S13 consumes 2seconds, as stated above. However, the other processing completes almostinstantaneously, so that the steps S1 through S12 or the steps S1through 13 consumes only a little over 2 seconds. On the other hand, thetoner content data is not updated over a little over 7 seconds, which isa single toner content calculating time. It follows that when thecontroller 202 repeatedly controls the toner content, the same tonercontent data is read out of the memory 203 and used at least threetimes.

[0204] As stated above, the controller 202 can execute the expectedcontrol based on the toner content data stored in the memory 203 withoutwaiting until the sensing controller 201 calculates a toner content.

[0205] Again, when the actual liquid level reaches the emergency-stopliquid level, it is likely that the paddle 116 bodily appears above theliquid surface and scatters the liquid 7. In this case, thereplenishment controller 202 stops the operation of the entire printer,although not shown in FIG. 30.

[0206] A thixotropic developing liquid having a toner content of 5% to40% and a viscosity of 100 mPa·s to 10,000 mPa·s decreases its viscositylittle by little to lower saturation viscosity when subjected to ashearing force. When the shearing force disappears, the liquid increasesits viscosity little by little to hardening saturation density. Tonercannot be sufficiently dispersed in such a liquid unless the controlagent is replenished with the viscosity of the liquid being evenlylowered by agitation in the second reservoir 115.

[0207] The illustrative embodiment, like the first embodiment,efficiently agitates the liquid 7 in the second reservoir 115, asdescribed with reference to FIG. 10. In addition, the illustrativeembodiment desirably collects the liquid from a cleaning blade andthereby further stabilizes the toner content of the liquid 7 in thesecond reservoir 115, as will be described hereinafter.

[0208] Assume that a cleaning blade is used to remove the thixotropicdeveloping liquid, which is dependent on a shearing force, from thedeveloping roller 105 or similar member. Then, the liquid removed by thecleaning blade moves along the surface of the blade due to gravity andthen drops from the blade due to gravity. At this instant, when theliquid moves away from the developing roller 105 to a position where theformer does not contact the latter, the viscosity of the liquid sharplyincreases because a shearing force derived from contact with the roller105 disappears. As the viscosity of the liquid increases, the fluidityof the liquid on the blade decreases. As aresult, the carrier of theliquid moves little by little with the toner depositing and cohering onthe blade.

[0209] The toner cohered on the cleaning blade blocks the flow of thefollowing liquid removed from the developing roller 105 and therebycauses the liquid to overflow via the end of the roller 105. Further, itis likely that the mass of the toner grown on the cleaning blade to acertain weight drops from the blade due to gravity and is then returnedto the second reservoir 115, making the toner content of the liquid 7 inthe reservoir 115 unstable. Moreover, such masses of toner are apt toreach the developing position without being loosened and adverselyeffect development.

[0210] To solve the above problems, the illustrative embodimentadditionally includes a shearing force exerting member for exerting ashearing force on the liquid moving on and along the cleaning blade dueto gravity. The shearing force applying member is applicable to any oneof the cleaning blade 4 a of the drum cleaner 4, the cleaning blade 112of the collecting section 102, and the cleaning blade 172 of thesweeping unit 170, FIG. 29.

[0211]FIG. 31 shows a blade 180 that is a specific form of the shearingforce exerting member. As shown, the blade 180 has an edge abuttingagainst any one of the cleaning blades 4 a, 112 and 172. Drive means,not shown, causes the blade 180 to move back and forth on and along thesurface of the cleaning blade in the lengthwise direction of thecleaning blade. The blade 180 moving back and forth exerts a shearingforce on the liquid 7 deposited on the cleaning blade, thereby causingthe viscosity of the liquid to increase little. This successfullyreduces the cohesion of the toner on the cleaning blade and therebyfurther stabilizes the toner content of the liquid 7 in the secondreservoir 115.

[0212]FIG. 32 shows a roller 181 that is another specific form of theshearing force exerting member. As shown, the roller 181 rotates whilecontacting substantially the entire length of the cleaning blade 4 a,112 or 172. The roller 181 is formed of sponge or similar elasticmaterial. The roller 181 rotates while sandwiching the liquid 7deposited on the cleaning blade between the roller 181 and the cleaningblade, thereby exerting a shearing force on the liquid 7. As a result,the viscosity of the liquid 7 increases little and prevents the tonerfrom cohering on the cleaning blade. This further stabilizes the tonercontent of the liquid 7 in the second reservoir 115. If desired, theroller 181 may be replaced with a paddle, a brush or a screw rotatablein contact with the cleaning blade over substantially the entire lengthof the blade.

Fifth Embodiment

[0213] A fifth embodiment of the present invention to be described isdirected mainly toward the fifth object stated earlier. This embodimentis essentially similar to the fourth embodiment. In the figures,identical structural elements are designated by identical referencenumerals and will not be described specifically in order to avoidredundancy. FIG. 33 shows arrangements for liquid collection unique tothe illustrative embodiment.

[0214] As shown in FIG. 33, the cleaning blade 4 a of the drum cleaner 4f removes the residual image transfer liquid from the drum 1. The screw4 b conveys the removed liquid horizontally until the liquid drops intothe residual liquid reservoir 160 via the collection pipe 8 due togravity. A conventional liquid level sensor, not shown, is positioned inthe residual liquid reservoir 160. When the liquid level sensor sensesthe upper-limit liquid level, a “reservoir full” error appears on, e.g.,a display. In the illustrative embodiment, the reservoir 160 isremovably mounted to the printer body. When the “reservoir full” errorappears on the display, the user of the printer removes the reservoir160 from the printer body, empties the reservoir 160, and again mountsthe reservoir 160 to the printer body. The residual liquid is thereforenot reused in the printer, but is collected by a trader concerned.

[0215] In the developing unit 100, the cleaning blade 112 scrapes offthe residual development liquid deposited on the collecting roller 111.The screw 113 conveys the residual development liquid horizontally untilthe liquid drops via the collection pipe 114 due to gravity. In theillustrative embodiment, the collection pipe 114 branches into a firstpath and a second path. The first path terminates at a residual liquidreservoir 164 via a solenoid-operated valve 162 while the second pathterminates at the second reservoir 115 via a solenoid-operated valve163.

[0216]FIG. 34 shows part of electric circuitry included in theillustrative embodiment. As shown, the control unit 200 includes animage controller 204 in addition to the sensing controller 201,replenishment controller 202, and memory 203. The solenoid-operatedvalves 162 and 163 are connected to the replenishment controller 202together with the conveyance pump 120, liquid pump 120, carrier pump147, agitation motor 148, and liquid level sensor 135.

[0217] The image controller 204 causes the optical writing unit, notshown, to form a latent image on the drum 1 in accordance with an imagesignal fed from a personal computer not shown. More specifically, theimage controller 204 calculates the image density of an image to beformed on the basis of the above image signal and delivers thecalculated image density to the replenishment controller 202 as imagedensity data. The image density refers to the ratio of an image area(total area of dots expected to form an image) to the entire area of thedrum 1 to which the liquid 7 will be applied. The replenishmentcontroller 202 drives the solenoid-operated valves 162 and 263 inaccordance with the image density data.

[0218] As for the residual development liquid collected from thecollecting roller 111, FIG. 33, the toner content is higher than, lowerthan or equal to one before development, depending on the image densityof an image to be formed. Specifically, when the image density reaches acertain value, the ratio of the amount of toner transferred from thedeveloping roller 105 to the drum 1 at the developing position to theamount of carrier liquid becomes substantially equal to the ratio of thesame in the liquid 7 before development. Consequently, the toner contentof the residual development liquid becomes equal to the toner content ofthe liquid 7 before development. The image density in this particularcondition will be referred to as equilibrium density hereinafter.

[0219] When the image density is higher than the equilibrium density,the ratio of the toner transferred from the developing roller 105 to thedrum 1 at the developing position becomes greater than the ratio of thetoner in the liquid 7 before development. Consequently, the tonercontent of the residual development liquid becomes lower than the tonercontent of the liquid 7 before development. Conversely, when the imagedensity is lower than the equilibrium density, the ratio of the tonertransferred to the developing roller 105 to the drum 1 decreases withthe result that the toner content of the residual development liquidbecomes higher than the toner content of the liquid 1 beforedevelopment. The memory 203 stores data representative of theequilibrium density determined by experiments.

[0220] The replenishment controller 202 compares the image density datareceived from the image controller 204 with the equilibrium density datastored in the memory 203. If the image density data is greater than theequilibrium density data, then the controller 202 causes thesolenoid-operated value 162 to open and causes the solenoid-operatedvalve 163 to close. If the image density data is not greater than theequilibrium density data, then the controller 202 causes the valve 162to close and causes the valve 163 to open. Consequently, the residualdevelopment liquid lower in toner content than before development iscollected in the residual liquid reservoir 164. Also, the residualdevelopment liquid higher in toner content than before development orequal thereto is not collected in the reservoir 164, but is returned tothe second reservoir 115 to be reused. At this instant, the valves 162and 163 and collection pipe 114 serve as bypass conveying means forreturning the liquid to the reservoir 115.

[0221] The solenoid-operated valves 162 and 163 may be replaced withmotor valves, if desired.

[0222] Assume that the residual development liquid is difficult to dropfrom the collecting section 102 into the second reservoir 115 or theresidual liquid reservoir 164 due to gravity. Then, as shown in FIG. 35,the bypass conveying means may be modified as shown in FIG. 35. Asshown, a collection pipe 114 a extends from the collecting section 102to the residual liquid reservoir 164 via a conveyance pump 165 a.Another collecting pipe 114 b extends from the collecting section 102 tothe second reservoir 115 via a conveyance pump 165 b. In this case, thereplenishment controller 202 drives either one of the pumps 165 a and165 b by comparing the image density data and equilibrium density,thereby delivering the residual development liquid to either one of thereservoirs 164 and 115.

[0223] In the illustrative embodiment, assume that when the liquid levelin the second reservoir 115 drops to allow the control agent to bereplenished, it is necessary to dilute the liquid 7 in the reservoir115. Then, the residual development liquid stored in the residual liquidreservoir 164 is replenished prior to the carrier liquid, as statedearlier. The residual liquid reservoir 164 is therefore prevented frombeing filled up with the residual development liquid. For safetypurpose, the illustrative embodiment additionally includes thepreviously mentioned liquid level sensor disposed in the residual liquidreservoir 164, which is removable from the printer body.

[0224] Assume that a “reservoir full” error representative of theupper-limit liquid level sensed by the above sensor appears on thedisplay, as stated previously. Then, the user removes the reservoir 164from the printer body, pours part of the liquid into another vessel, andagain mounts the reservoir 164 to the printer body to thereby cancel the“reservoir full” error. Subsequently, when the liquid level in thereservoir 164 drops to a sufficient level, the user returns the liquidfrom the vessel to the reservoir 164 for a recycling purpose.

[0225] The illustrative embodiment, like the fourth embodiment, uses atarget toner content higher than the standard toner content (15 wt %),but lower than or equal to the upper limit (18 wt %) of the allowablerange. It is therefore possible to obviate a sharp decrease in the tonercontent of the liquid 7 and therefore to obviate short image densityascribable to short toner content.

[0226] However, the equilibrium density is not image density thatimplements the standard toner content (15 wt %), but is image densitythan can make the toner content of the residual development liquid equalto that of the liquid 7 before development. Moreover, the equilibriumdensity varies along with the toner content of the liquid 7 beforedevelopment, i.e., stored in the second reservoir 115. FIG. 36 shows aspecific relation between the equilibrium density and the toner contentof the liquid 7 before development. As shown, when the toner content ofthe liquid 7 is coincident with the upper-limit toner content, theequilibrium density is 35% while the toner content of the residualliquid is 18 wt %, which is the upper limit. Likewise, when the tonercontent of the liquid 7 is coincident with the lower-limit tonercontent, the equilibrium density is 40% while the toner content of theresidual liquid is 12 wt %, which is the lower limit. Further, when thetoner content of the liquid 7 is coincident with the standard tonercontent, the equilibrium density is 37.5% while the toner content of theresidual liquid is 15 wt %, which is the standard toner content.

[0227] In the illustrative embodiment, the memory 203 stores a datatable showing correspondence between various toner contents andequivalent densities mentioned above. The replenishment controller 202selects a particular equilibrium density by comparing the toner contentof the liquid 7 and data table.

[0228] The control scheme stated above, however, cannot solve thefollowing problem alone. Assume that the toner content of the liquid 7is coincident with the upper limit (18 wt %) of the allowable range, andthat the image density data is greater than the equilibrium density.Then, the residual development liquid short of the upper limit is storedin the residual liquid reservoir 164. On the other hand, assume that thetoner content of the liquid 7 is coincident with the lower limit (12 wt%) of the allowable range, and that the image density data is greaterthan the equilibrium density. Then, the residual development liquidshort of the lower limit is stored in the reservoir 164. It is thereforelikely that not only the residual development liquid whose toner densityis lower than the standard density (15 wt %), but also the residualdevelopment liquid whose toner density is above the standard density andcan be directly returned to the second reservoir 115 are stored in theresidual liquid reservoir 164.

[0229] The above control scheme therefore causes the residualdevelopment liquid capable of being recycled to be simply wasted. Assumethat the residual development liquid stored in the liquid tank 164 isused as a control agent for diluting the liquid 7 in the secondreservoir 115, as in the fourth embodiment. Then, the control agentincreases the toner content of the residual development liquid above thestandard toner density and cannot adequately implement adjustment. It istherefore preferable to set for each toner content of the liquid 7 aparticular equilibrium density that does not lower the toner content ofthe residual development liquid below the toner content of the liquid 7in the reservoir 115, but lowers it below the standard toner content (15wt %). This allows only the residual development liquid whose tonercontent is lower than the standard content to be stored in the residualliquid reservoir 164.

[0230] In the illustrative embodiment, the replenishment controller 202writes in the memory 203 not only the image density data of a new imageto be formed, but also the image density data of a plurality of imagesformed in the past. Specifically, the controller 202 writes the imagedensity data of the last nineteen images in the memory 203 in additionto the image density data of a new image to be formed.

[0231] Furthermore, two different target toner densities are set in therange higher than the target toner density (15 wt %), but lower than orequal to the upper limit of the allowable range (18 wt % or below). Thereplenishment controller 202 produces a mean value of the twentyconsecutive image density data in total and compares it with theequilibrium density. If the mean value is greater than the equilibriumvalue, then the controller 202 selects greater one of the target tonerdensities; if otherwise, it selects smaller one of the target tonerdensities.

[0232] The mean value of the past image density data read out of thememory 203 shows whether or not the user tends to continuously produceimages with relatively high image density. If the answer of thisdecision is positive, then the replenishment controller 202 selects thehigher target density than usual. Therefore, even when images withrelatively high density are continuously produced for a moment, thecontroller 202 surely prevents the toner content of the liquid 7 in thesecond reservoir 115 from falling below the lower limit of the allowablerange and lowering image density.

[0233] As stated above, in the illustrative embodiment, the residualdevelopment liquid whose toner content is equal to or higher than onebefore development is returned to the second reservoir 115 and reused.On the other hand, the residual development liquid whose toner contentis lower than one before development is stored in the residual liquidreservoir 164. Consequently, the toner content of the liquid 7 in thesecond reservoir 155 remains stable more than conventional.

[0234] The illustrative embodiment, too, should preferably be providedwith the implementation for efficiently agitating the liquid 7 in thesecond reservoir 115 and an implementation for efficiently collectingthe liquid removed by the cleaning blades.

[0235] Modifications of the illustrative embodiment will be describedhereinafter. FIG. 37 shows a modification additionally including arecycling pipe 167 extending from the residual liquid reservoir 164 tothe second reservoir 115 via a residual liquid pump 166. In themodification, the memory 203 stores values, which lower the tonerdensity of the liquid 7 in the reservoir 115 below the standard tonerdensity (15 wt %), in correspondence to the toner densities of theliquid 7 as equilibrium values. Therefore, only the residual developmentliquid whose toner content is lower than the standard density is storedin the residual liquid reservoir 164.

[0236] In this modification, the replenishment controller 202 executesthe following control when determining that the liquid 7 in the secondreservoir 115 should be diluted. First, the controller 202 drives theresidual liquid pump 166 for a preselected period of time in order todeliver the residual development liquid from the residual liquidreservoir 164 to the second reservoir 155. If the liquid level in thesecond reservoir 155 does not rise to the standard level, as describedin relation to the fourth embodiment, in the above period of time, thenthe controller 202 stops driving the residual liquid pump 166 and startsdriving the carrier pump 147.

[0237] With the above control, the controller 202 temporarily stores theresidual development liquid whose toner content has decreased below thestandard toner content (15 wt %) in the residual liquid reservoir 164.The controller 202 then returns the residual development liquid to thesecond reservoir 115 as a control agent for diluting the liquid 7, asneeded. Therefore, the entire residual development liquid collected canbe recycled without being discarded. Further, the residual developmentliquid stored in the reservoir 164 is used prior to the carrier liquidas a control agent. This causes a minimum of carrier liquid to bewastefully replenished and further promotes the efficient recycling ofthe residual development liquid.

[0238]FIG. 38 shows another modification of the illustrative embodiment.As shown, the collection pipe 8 connected at one end to the residualliquid reservoir 160 is connected at the other end to the portion of thecollection pipe 114 of the collecting portion 102 closer to the section102 than to the solenoid-operated valves 162 and 163. In thisconfiguration, a mixture of the residual development liquid and residualimage transfer liquid is collected in the collection pipe 114 and thenreturned to the second reservoir 115 or collected in a mixture reservoir168, which is a substitute for the residual liquid reservoir 164. Amixture pump 169 returns the mixture collected in the mixture reservoir168 to the second reservoir 115.

[0239] Assume that a toner image formed on the drum 1 is directlytransferred to the paper sheet P without the intermediately of therecording medium 3, FIG. 28. Then, the quantity of residual liquid to becollected from the drum 1 is dependent on the liquid absorbability ofthe recording medium P. For example, a greater amount of liquid iscollected when the recording medium P is a porous paper sheet with highabsorbability than when it is an OHP (OverHead Projector) sheet with lowabsorbability.

[0240] By contrast, when a toner image is transferred from the drum 1 tothe recording medium P via the intermediate drum 3 or any otherintermediate image transfer body, as in the modification, theintermediate drum 3 absorbs a constant amount of liquid and thereforestabilizes the amount of liquid to be collected. However, the amount ofliquid to be collected is not constant. In this configuration, the tonercontent of the mixture of the residual liquids is correlated to imagedensity. When image density has a certain constant value, the tonercontent of the above mixture is coincident with the standard tonercontent (15 wt %). Let such image density be referred to as a mixtureequilibrium value.

[0241] In the modification, the memory 203 stores a data table showingcorrespondence between experimentally determined mixture equilibriumvalues and the toner contents of the liquid 7 in the second reservoir115. The replenishment controller 202 scans the data table to select onemixture equilibrium value corresponding to the toner content of theliquid 7 in the reservoir 115 and compares it with image density datareceived from the image controller 204. If the image density data isgreater than the mixture equilibrium value, then the replenishmentcontroller 202 opens the valve 162 and closes the valve 163; ifotherwise, it closes the valve 162 and opens the valve 163.Consequently, the mixture whose standard toner density is also loweredis stored in the mixture tank 168. On the other hand, the mixture whosetoner content is equal to or higher than the standard toner density isreturned to the reservoir 115 and recycled.

[0242] Assume that the replenishment controller 202 determines that theliquid 7 in the reservoir 115 should be diluted. Then, the controller202 drives the collection pump 169 for a preselected period of time. Ifthe liquid level in the reservoir 115 does not rise to the standardliquid level in the above period of time, then the controller 202 stopsdriving the collection pump 169 and starts driving the carrier pump 147.

[0243] The modifications shown and described each are capable ofrecycling not only the liquid left after development but also the liquidleft after image transfer.

Sixth Embodiment

[0244] A sixth embodiment to be described is directed mainly toward thefifth object stated earlier. The basic configuration of this embodimentis identical with the configuration of the fourth embodiment and willnot be described specifically in order to avoid redundancy. As shown inFIG. 39, the residual image transfer liquid in the drum cleaner and theresidual development liquid in the collecting section 102 are directlyreturned to the second reservoir 115 without the intermediary of anyintermediate reservoir. The illustrative embodiment, like the fifthembodiment, includes the image controller 204, FIG. 34.

[0245] As for the mixture liquid referred to in the fifth embodiment,the toner content can be adjusted if the timing for forming a latentimage on the drum 1 is varied and if the developing time is variedaccordingly. Specifically, the liquid left on part of the developingroller 105 contacted the non-image area of the drum 1 has a higher tonercontent than before development. This is because the liquid left on thedeveloping roller 105 after development causes toner to migrate to thedrum 1 little, but causes some carrier liquid to deposit on the drum 1due to its viscosity.

[0246] On the other hand, the developing roller 105 sequentiallyexecutes development with the drum 1 from a front non-image portion(upper margin of the recording medium P and zone preceding it) to a rearnon-image portion (lower margin of the recording medium P and zonefollowing it) via an image portion between the upper and lower marginsin the circumferential direction. Hardly any toner deposits on the frontand rear non-image portions because a latent image is absent there.

[0247] By varying the latent image forming timing such that the frontnon-image portion varies in length in the circumferential direction ofthe drum 1, it is possible to vary the amount of residual liquid whosetoner content is higher than before development and therefore to varythe toner content of the mixture liquid. It follows that even when asolid image is formed over the entire recording medium P, the tonercontent of the mixture liquid can remain above the standard tonerdensity if the latent image forming timing is delayed by a large marginin accordance with the image density of the solid image.

[0248] In light of the above, the image controller 204 calculates theimage density of an image to be formed and varies the latent imageforming timing in accordance with the calculated image density.Consequently, the mixture liquid has a toner content equal to or abovethe toner content (15 wt %) without regard to image density.

[0249] The illustrative embodiment does not return the mixture liquidwhose toner content is lower than the standard toner content to thesecond reservoir 115 and thereby maintains the toner content of theliquid 7 in the reservoir 115 stable more than conventional.

[0250] The fourth to sixth embodiments shown and described have variousunprecedented advantages, as enumerated below.

[0251] (1) The developing liquid in the liquid storing section has itstoner content stabilized more than conventional.

[0252] (2) The residual image transfer liquid is returned to the liquidstoring section only when needed, so that the liquid in the storingsection has its toner content stabilized more than conventional.

[0253] (3) The residual development liquid that will dilute thedeveloping liquid is stored in the residual liquid storing section. Thisalso stabilizes the toner content of the liquid in the liquid storingsection more than conventional.

[0254] (4) The liquid in the residual liquid storing section can berecycled as a control agent for diluting the developing liquid in theliquid storing section.

[0255] (5) Not only the residual development liquid but also theresidual image transfer liquid can be used as a control agent.

[0256] (6) Even when the mixture liquid or control agent is short, thecarrier liquid can be used as a control agent in order to adequatelydilute the developing liquid.

[0257] (7) The residual image transfer liquid and residual developmentliquid can be efficiently recycled without being wastefully stored.

[0258] (8) The toner content of the liquid in the liquid storing sectioncan be restored to the target content not only when the toner content islowered below the target content but also when the former is raisedabove the latter.

[0259] (9) There can be obviated the fall of image density ascribable tothe dilution of the liquid in the liquid storing section beyond theallowable range.

[0260] (10) Even when images with relatively high image density arecontinuously output for a moment, there can be obviated the fall ofimage density ascribable to the dilution of the liquid in the liquidstoring section beyond the allowable range.

[0261] (11) A latent image forming timing is adjusted in accordance withthe image density of an image to be formed. This is also successful tostabilize the substance content of the liquid in the liquid storingsection more than conventional.

[0262] (12) Even when the developing liquid is thixotropic, a quantityof light representative of the substance content of the liquid is surelyobtained from the liquid. This allows the substance content of theliquid to be surely sensed.

[0263] (13) A liquid level in the liquid storing section can be confinedin an adequate range.

[0264] (14) A difference between the actual liquid level in the liquidstoring section and the liquid level sensed by liquid level sensingmeans is reduced.

[0265] (15) Images with high density can be formed by a smaller amountof liquid than when the liquid has a substance content lower than 5%.Further, images with high density can be formed at lower cost than whenuse is made of an expensive developing liquid whose viscosity is above10,000 mPa·s for easy agitation. Moreover, irregular image densityascribable to the irregular dispersion of toner can be reduced more thanwhen use is made of a developing liquid whose viscosity is lower than100 mPa·s and apt to bring about the above irregular dispersion.

[0266] Various modifications will become possible for those skilled inthe art after receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. A developing device for developing a latent imageformed on an image carrier with a developing liquid, said developingdevice comprising: a liquid storing section for storing the developingliquid consisting of a developing substance and a carrier liquid; adeveloper carrier for depositing the developing liquid fed from saidliquid storing section thereon; and a float type liquid level sensor forsensing a liquid level in said liquid storing section; said float typeliquid level sensor comprising: a magnetic force generating member;magnetic force sensing means for sensing a magnetic force of saidmagnetic force generating member; and spherical floats movable in anup-and-down direction in accordance with the liquid level in said liquidstoring section, wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 2. The device as claimed in claim 1, wherein said magnetic forcegenerating member is movable up and down relative to said magnetic forcesensing means together with said floats.
 3. The device as claimed inclaim 2, wherein said magnetic force sensing means comprises a pluralityof magnetic force sensing means arranged in parallel in the up-and-downdirection.
 4. The device as claimed in claim 3, wherein said floats areformed of foam resin.
 5. The device as claimed in claim 4, furthercomprising agitating means disposed in said liquid storing section androtatable about an axis offset from a center of a cross-section of saidliquid storing section for agitating the developing liquid.
 6. Thedevice as claimed in claim 5, wherein said agitating means comprises: aflexible paddle rotatable in contact with an inner periphery of saidliquid storing section while deforming itself; and a non-flexible paddlerotatable about a same axis of rotation as said flexible paddle forcausing the developing liquid to flow along said axis.
 7. The device asclaimed in claim 6, wherein said non-flexible paddle causes thedeveloping liquid to flow downward toward a bottom of said liquidstoring section along the axis of rotation.
 8. The device as claimed inclaim 7, wherein said liquid storing section is formed with a taper at abottom corner such that said taper guides the developing liquid flowndownward and rebounded from the bottom upward in a direction opposite toa direction in which the axis is offset from the center.
 9. The deviceas claimed in claim 1, wherein said floats are formed of foam resin. 10.The device as claimed in claim 9, further comprising agitating meansdisposed in said liquid storing section and rotatable about an axisoffset from a center of a cross-section of said liquid storing sectionfor agitating the developing liquid.
 11. The device as claimed in claim10, wherein said agitating means comprises: a flexible paddle rotatablein contact with an inner periphery of said liquid storing section whiledeforming itself; and a non-flexible paddle rotatable about a same axisof rotation as said flexible paddle for causing the developing liquid toflow along said axis.
 12. The device as claimed in claim 11, whereinsaid non-flexible paddle causes the developing liquid to flow downwardtoward a bottom of said liquid storing section along the axis ofrotation.
 13. The device as claimed in claim 12, wherein said liquidstoring section is formed with a taper at a bottom corner such that saidtaper guides the developing liquid flown downward and rebounded from thebottom upward in a direction opposite to a direction in which the axisis offset from the center.
 14. The device as claimed in claim 1, furthercomprising agitating means disposed in said liquid storing section androtatable about an axis offset from a center of a cross-section of saidliquid storing section for agitating the developing liquid.
 15. Thedevice as claimed in claim 14, wherein said agitating means comprises: aflexible paddle rotatable in contact with an inner periphery of saidliquid storing section while deforming itself; and a non-flexible paddlerotatable about a same axis of rotation as said flexible paddle forcausing the developing liquid to flow along said axis.
 16. The device asclaimed in claim 15, wherein said non-flexible paddle causes thedeveloping liquid to flow downward toward a bottom of said liquidstoring section along the axis of rotation.
 17. The device as claimed inclaim 16, wherein said liquid storing section is formed with a taper ata bottom corner such that said taper guides the developing liquid flowndownward and rebounded from the bottom upward in a direction opposite toa direction in which the axis is offset from the center.
 18. An imageforming apparatus comprising: an image carrier for forming a latentimage thereon; image forming means for forming the latent image on saidimage carrier; and a developing device for depositing a developingsubstance contained in a developing liquid on the latent image tothereby develop said latent image; said developing device comprising: aliquid storing section for storing the developing liquid consisting ofthe developing substance and a carrier liquid; a developer carrier fordepositing the developing liquid fed from said liquid storing sectionthereon; and a float type liquid level sensor for sensing a liquid levelin said liquid storing section; said float type liquid level sensorcomprising: a magnetic force generating member; magnetic force sensingmeans for sensing a magnetic force of said magnetic force generatingmember; and spherical floats movable in an up-and-down direction inaccordance with the liquid level in said liquid storing section; whereina distance between said magnetic force generating member and saidmagnetic force sensing means varies in accordance with a movement ofsaid floats, allowing the liquid level to be determined on the basis ofan output of said magnetic force sensing means.
 19. The apparatus asclaimed in claim 18, further comprising: content sensing means forsensing a content of the developing substance of the developing liquidstored in said liquid storing section; collecting means for collectingthe developing liquid used for development and returning said developingliquid to said liquid storing section; replenishing means forreplenishing a control agent for controlling the content of thedeveloping substance of the developing liquid to said liquid storingsection; and control means for controllably driving said replenishingmeans in accordance with an output of said content sensing means and anoutput of said liquid level sensor to thereby control the content of thedeveloping substance.
 20. The apparatus as claimed in claim 19, whereinwhether or not the control agent is present in said replenishing meansis determined on the basis of the output of said liquid level sensor anda duration of drive of said replenishing means.
 21. The apparatus asclaimed in claim 20, wherein the developing liquid contains thedeveloping substance dispersed in a content of between 5% and 40% andhas viscosity of between 100 mPa·s and 10,000 mPa·s.
 22. A developingdevice for developing a latent image formed on an image carrier with adeveloping liquid, said developing device comprising: a liquid storingsection for storing the developing liquid consisting of a developingsubstance and a carrier liquid; a developer carrier for depositing thedeveloping liquid fed from said liquid storing section thereon; and afloat type liquid level sensor for sensing a liquid level in said liquidstoring section; said float type liquid level sensor comprising: amagnetic force generating member; magnetic force sensing means forsensing a magnetic force of said magnetic force generating member;floats movable in an up-and-down direction in accordance with the liquidlevel in said liquid storing section; an elongate, ring support membersupporting said floats at opposite ends thereof, supporting either oneof said magnetic force generating member and said magnetic force sensingmeans at a position intermediate between said opposite ends, andsupporting rings between said position and said opposite ends; and aplurality of guide rods respectively inserted in said rings for guidingsaid floats in the up-and-down direction; wherein a distance betweensaid magnetic force generating member and said magnetic force sensingmeans varies in accordance with a movement of said floats, allowing theliquid level to be determined on the basis of an output of said magneticforce sensing means.
 23. The device as claimed in claim 22, wherein saidmagnetic force generating member is movable up and down relative to saidmagnetic force sensing means together with said floats.
 24. The deviceas claimed in claim 23, wherein said magnetic force sensing meanscomprises a plurality of magnetic force sensing means arranged inparallel in the up-and-down direction.
 25. The device as claimed inclaim 24, wherein said floats are formed of foam resin.
 26. The deviceas claimed in claim 25, further comprising agitating means disposed insaid liquid storing section and rotatable about an axis offset from acenter of a cross-section of said agent storing section for agitatingthe developing liquid.
 27. The device as claimed in claim 26, whereinsaid agitating means comprises: a flexible paddle rotatable in contactwith an inner periphery of said liquid storing section while deformingitself; and a non-flexible paddle rotatable about a same axis ofrotation as said flexible paddle for causing the developing liquid toflow along said axis.
 28. The device as claimed in claim 27, whereinsaid non-flexible paddle causes the developing liquid to flow downwardtoward a bottom of said liquid storing section along the axis ofrotation.
 29. The device as claimed in claim 28, wherein said liquidstoring section is formed with a taper at a bottom corner such that saidtaper guides the developing liquid flown downward and rebounded from thebottom upward in a direction opposite to a direction in which the axisis offset from the center.
 30. The device as claimed in claim 22,wherein said floats are formed of foam resin.
 31. The device as claimedin claim 30, further comprising agitating means disposed in said liquidstoring section and rotatable about an axis offset from a center of across-section of said liquid storing section for agitating thedeveloping liquid.
 32. The device as claimed in claim 31, wherein saidagitating means comprises: a flexible paddle rotatable in contact withan inner periphery of said liquid storing section while deformingitself; and a non-flexible paddle rotatable about a same axis ofrotation as said flexible paddle for causing the developing liquid toflow in along said axis.
 33. The device as claimed in claim 32, whereinsaid non-flexible paddle causes the developing liquid to flow downwardtoward a bottom of said liquid storing section along the axis ofrotation.
 34. The device as claimed in claim 33, wherein said liquidstoring section is formed with a taper at a bottom corner such that saidtaper guides the developing liquid flown downward and rebounded from thebottom upward in a direction opposite to a direction in which the axisis offset from the center.
 35. The device as claimed in claim 22,further comprising agitating means disposed in said liquid storingsection and rotatable about an axis offset from a center of across-section of said liquid storing section for agitating thedeveloping liquid.
 36. The device as claimed in claim 35, wherein saidagitating means comprises: a flexible paddle rotatable in contact withan inner periphery of said liquid storing section while deformingitself; and a non-flexible paddle rotatable about a same axis ofrotation as said flexible paddle for causing the developing liquid toflow along said axis.
 37. The device as claimed in claim 36, whereinsaid non-flexible paddle causes the developing liquid to flow downwardtoward a bottom of said liquid storing section along the axis ofrotation.
 38. The device as claimed in claim 37, wherein said liquidstoring section is formed with a taper at a bottom corner such that saidtaper guides the developing liquid flown downward and rebounded from thebottom upward in a direction opposite to a direction in which the axisis offset from the center.
 39. An image forming apparatus comprising: animage carrier for forming a latent image thereon; image forming meansfor forming the latent image on said image carrier; and a developingdevice for depositing a developing substance contained in a developingliquid on the latent image to thereby develop said latent image; saiddeveloping device comprising: a liquid storing section for storing thedeveloping liquid consisting of the developing substance and a carrierliquid; a developer carrier for depositing the developing liquid fedfrom said liquid storing section thereon; and a float type liquid levelsensor for sensing a liquid level in said liquid storing section; saidfloat type liquid level sensor comprising: a magnetic force generatingmember; magnetic force sensing means for sensing a magnetic force ofsaid magnetic force generating member; floats movable in an up-and-downdirection in accordance with the liquid level in said liquid storingsection; and an elongate, ring support member supporting said floats atopposite ends thereof, supporting either one of said magnetic forcegenerating member and said magnetic force sensing means at a positionintermediate between said opposite ends, and supporting rings betweensaid position and said opposite ends; and a plurality of guide rodsrespectively inserted in said rings for guiding said floats in theup-and-down direction; wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 40. The apparatus as claimed in claim 39, further comprising:content sensing means for sensing a content of the developing substanceof the developing liquid stored in said liquid storing section;collecting means for collecting the developing liquid used fordevelopment and returning said developing liquid to said liquid storingsection; replenishing means for replenishing a control agent forcontrolling the content of the developing substance of the developingliquid to said liquid storing section; and control means forcontrollably driving said replenishing means in accordance with anoutput of said content sensing means and an output of said liquid levelsensor to thereby control the content of the developing substance. 41.The apparatus as claimed in claim 40, wherein whether or not the controlagent is present in said replenishing means is determined on the basisof the output of said liquid level sensor and a duration of drive ofsaid replenishing means.
 42. The apparatus as claimed in claim 41,wherein the developing liquid contains the developing substancedispersed in a content of between 5% and 40% and has viscosity ofbetween 100 mPa·s and 10,000 mPa·s.
 43. A developing device fordeveloping a latent image formed on an image carrier with a developingliquid, said developing device comprising: a liquid storing section forstoring the developing liquid consisting of a developing substance and acarrier liquid; a developer carrier for depositing the developing liquidfed from said liquid storing section thereon; and agitating meansrotatable in said liquid storing section for causing the developingliquid to swirl in a horizontal direction to thereby agitate saiddeveloping liquid; wherein said agitating means rotates about an axisoffset from a center of a cross-section of said liquid storing section.44. The device as claimed in claim 43, wherein said agitating meanscomprises: a flexible paddle rotatable in contact with an innerperiphery of said liquid storing section while deforming itself; and anon-flexible paddle rotatable about a same axis of rotation as saidflexible paddle for causing the developing liquid to flow along saidaxis.
 45. The device as claimed in claim 44, wherein said non-flexiblepaddle causes the developing liquid to flow downward toward a bottom ofsaid liquid storing section along the axis of rotation.
 46. The deviceas claimed in claim 45, wherein said liquid storing section is formedwith a taper at a bottom corner such that said taper guides thedeveloping liquid flown downward and rebounded from the bottom upward ina direction opposite to a direction in which the axis is offset from thecenter.
 47. The device as claimed in claim 46, further comprising:collecting means for collecting the developing liquid used fordevelopment from said developer carrier and causing said developingliquid collected to drop to a liquid surface in said liquid storingsection at a position closer to the axis than to the center; contentsensing means for sensing a content of the developing substance of thedeveloping liquid guided upward by said taper; and content controllingmeans for causing a content agent to drop to the liquid surface inaccordance with an output of said content sensing means to therebycontrol a content of the developing substance of the developing liquidstored in said liquid storing section.
 48. An image forming apparatuscomprising: an image carrier for forming a latent image thereon; imageforming means for forming the latent image on said image carrier; and adeveloping device for depositing a developing substance contained in adeveloping liquid on the latent image to thereby develop said latentimage; said developing device comprising: a liquid storing section forstoring the developing liquid consisting of the developing substance anda carrier liquid; a developer carrier for depositing the developingliquid fed from said liquid storing section thereon; and agitating meansrotatable in said liquid storing section for causing the developingliquid to swirl in a horizontal direction to thereby agitate saiddeveloping liquid; wherein said agitating means rotates about an axisoffset from a center of a cross-section of said liquid storing section.49. The apparatus as claimed in claim 48, wherein said agitating meanscomprises: a flexible paddle rotatable in contact with an innerperiphery of said liquid storing section while deforming itself; and anon-flexible paddle rotatable about a same axis of rotation as saidflexible paddle for causing the developing liquid to flow along saidaxis.
 50. The apparatus as claimed in claim 49, wherein saidnon-flexible paddle causes the developing liquid to flow downward towarda bottom of said liquid storing section along the axis of rotation. 51.The apparatus as claimed in claim 50, wherein said liquid storingsection is formed with a taper at a bottom corner such that said taperguides the developing liquid flown downward and rebounded from thebottom upward in a direction opposite to a direction in which the axisis offset from the center.
 52. The apparatus as claimed in claim 51,further comprising: collecting means for collecting the developingliquid used for development from said developer carrier and causing saiddeveloping liquid collected to drop to a liquid surface in said liquidstoring section at a position closer to the axis than to the center;content sensing means for sensing a content of the developing substanceof the developing liquid guided upward by said taper; and contentcontrolling means for causing a content agent to drop to the liquidsurface in accordance with an output of said content sensing means tothereby control a content of the developing substance of the developingliquid stored in said liquid storing section.
 53. A developing devicefor developing a latent image formed on an image carrier with adeveloping liquid, said developing device comprising: a liquid storingsection for storing the developing liquid consisting of a developingsubstance and a carrier liquid; a developer carrier for depositing thedeveloping liquid fed from said liquid storing section thereon; andagitating means rotatable in said liquid storing section for causing thedeveloping liquid to swirl in a horizontal direction to thereby agitatesaid developing liquid, said agitating means comprising flexibleagitating means.
 54. The device as claimed in claim 53, wherein saidliquid storing section comprises: a first liquid storing section forstoring the developing liquid to be fed to said developer carrier; and asecond liquid storing section for storing a developing liquid to be fedto said first liquid storing section; said developing device furthercomprising: collecting means for collecting the developing liquid usedfor development from said developer carrier and feeding said developingliquid collected to said second liquid storing section; content sensingmeans for sensing a content of the developing substance of thedeveloping liquid stored in said second liquid storing section; andcontent controlling means for controlling the content of the developingsubstance in accordance with an output of said content sensing means;wherein said flexible agitating means is disposed at least in saidsecond liquid storing section.
 55. The device as claimed in claim 54,wherein said agitating means further comprises non-flexible agitatingmeans located at a position different from said flexible agitatingmeans.
 56. The device as claimed in claim 55, wherein said flexibleagitating means and said non-flexible agitating means are rotated by asingle shaft.
 57. The device as claimed in claim 56, wherein saidnon-flexible agitating means causes the developing liquid to flowdownward toward a bottom of said liquid storing section along the axisof rotation while swirling in accordance with the rotation of saidnon-flexible agitating means.
 58. The device as claimed in claim 57,wherein said liquid storing section comprises: a first liquid storingsection for storing the developing liquid to be fed to said developercarrier; and a second liquid storing section for storing a developingliquid to be fed to said first liquid storing section; said developingdevice further comprising: collecting means for collecting thedeveloping liquid used for development from said developer carrier andfeeding said developing liquid collected to said second liquid storingsection; content sensing means for sensing a content of the developingsubstance of the developing liquid stored in said second liquid storingsection; and content controlling means for controlling the content ofthe developing substance in accordance with an output of said contentsensing means; wherein said flexible agitating means is disposed atleast in said second liquid storing section.
 59. An image formingapparatus comprising: an image carrier for forming a latent imagethereon; image forming means for forming a latent image on said imagecarrier; and a developing device for developing the latent image bydepositing a developing substance contained in the developing liquid onsaid latent image; said developing device comprising: a liquid storingsection for storing the developing liquid consisting of the developingsubstance and a carrier liquid; a developer carrier for depositing thedeveloping liquid fed from said liquid storing section thereon; andagitating means rotatable in said liquid storing section for causing thedeveloping liquid to swirl in a horizontal direction to thereby agitatesaid developing liquid, said agitating means comprising flexibleagitating means.
 60. The apparatus as claimed in claim 59, wherein saidliquid storing section comprises: a first liquid storing section forstoring the developing liquid to be fed to said developer carrier; and asecond liquid storing section for storing a developing liquid to be fedto said first liquid storing section; said developing device furthercomprising: collecting means for collecting the developing liquid usedfor development from said developer carrier and feeding said developingliquid collected to said second liquid storing section; content sensingmeans for sensing a content of the developing substance of thedeveloping liquid stored in said second liquid storing section; andcontent controlling means for controlling the content of the developingsubstance in accordance with an output of said content sensing means;wherein said flexible agitating means is disposed at least in saidsecond liquid storing section.
 61. The apparatus as claimed in claim 60,wherein said agitating means further comprises non-flexible agitatingmeans located at a position different from said flexible agitatingmeans.
 62. The apparatus as claimed in claim 61, wherein said flexibleagitating means and said non-flexible agitating means are rotated by asingle shaft.
 63. The apparatus as claimed in claim 62, wherein saidnon-flexible agitating means causes the developing liquid to flowdownward toward a bottom of said liquid storing section along the axisof rotation while swirling in accordance with the rotation of saidnon-flexible agitating means.
 64. The apparatus as claimed in claim 63,wherein said liquid storing section comprises: a first liquid storingsection for storing the developing liquid to be fed to said developercarrier; and a second liquid storing section for storing a developingliquid to be fed to said first liquid storing section; said developingdevice further comprising: collecting means for collecting thedeveloping liquid used for development from said developer carrier andfeeding said developing liquid collected to said second liquid storingsection; content sensing means for sensing a content of the developingsubstance of the developing liquid stored in said second liquid storingsection; and content controlling means for controlling the content ofthe developing substance in accordance with an output of said contentsensing means; wherein said flexible agitating means is disposed atleast in said second liquid storing section.
 65. A developing device fordeveloping a latent image formed on an image carrier with a developingliquid, said developing device comprising: a liquid storing section forstoring the developing liquid consisting of a developing substance and acarrier liquid; a developer carrier for depositing the developing liquidfed from said liquid storing section thereon; and agitating meansrotatable in said liquid storing section for causing the developingliquid to swirl along an inner periphery of said liquid storing sectionto thereby agitate said developing liquid, said agitating meansgenerating a flow of said developing liquid along an axis of rotation ofsaid agitating means.
 66. The device as claimed in claim 65, wherein theflow of the developing liquid extends toward a bottom of said liquidstoring section.
 67. The device as claimed in claim 66, wherein saidagitating means comprises a plurality of blades radially extending froma shaft and positioned one above the other in an axial direction of saidshaft, said plurality of blades facing each other at a preselecteddistance while being inclined relative to said axial direction each. 68.The device as claimed in claim 67, wherein upper blades included in saidblades each have a rear portion in a direction of rotation that isinclined more sharply than a front portion in such a manner as toapproach a lower blade more than said front portion.
 69. The device asclaimed in claim 68, wherein said upper blade and said lower bladefacing each other are shifted from each other such that the frontportion and the rear portion of said upper blade are positioned ahead ofthe front portion and the rear portion of said lower blade.
 70. An imageforming apparatus comprising: an image carrier for forming a latentimage thereon; image forming means for forming a latent image on saidimage carrier; and a developing device for developing the latent imageby depositing a developing substance contained in a developing liquid onsaid latent image; said developing device comprising: a liquid storingsection for storing the developing liquid consisting of the developingsubstance and a carrier liquid; a developer carrier for depositing thedeveloping liquid fed from said liquid storing section thereon; andagitating means rotatable in said liquid storing section for causing thedeveloping liquid to swirl along an inner periphery of said liquidstoring section to thereby agitate said developing liquid, saidagitating means generating a flow of said developing liquid along anaxis of rotation of said agitating means.
 71. The apparatus as claimedin claim 70, wherein the flow of the developing liquid extends toward abottom of said liquid storing section.
 72. The apparatus as claimed inclaim 71, wherein said agitating means comprises a plurality of bladesradially extending from a shaft and positioned one above the other in anaxial direction of said shaft, said plurality of blades facing eachother at a preselected distance while being inclined relative to saidaxial direction each.
 73. The apparatus as claimed in claim 72, whereinupper blades included in said blades each have a rear portion in adirection of rotation that is inclined more sharply than a front portionin such a manner as to approach a lower blade more than said frontportion.
 74. The apparatus as claimed in claim 73, wherein said upperblade and said lower blade facing each other are shifted from each othersuch that the front portion and the rear portion of said upper blade arepositioned ahead of the front portion and the rear portion of said lowerblade.
 75. An image forming apparatus comprising: an image carrier forforming a latent image thereon; a liquid storing section for storing adeveloping liquid consisting of a developing substance and a carrierliquid; a developer carrier for depositing the developing liquid fedfrom said liquid storing section thereon and causing said developingliquid to deposit on the latent image to thereby develop said latentimage; transferring means for transferring a developed image from saidimage carrier to a recording medium; cleaning means for removing thedeveloping liquid left on said image carrier after image transfer;content sensing means for sensing a content of the developing substanceof the developing liquid stored in said liquid storing section; contentcontrolling means for comparing an output of said content sensing meansand a preselected target content and feeding a control agent to saidliquid storing section in accordance with a result of comparison tothereby control a content of the developing substance; and a residualliquid storing section for storing the developing liquid removed by saidcleaning means; wherein the developing liquid stored in said residualliquid storing section is used as the control agent.
 76. The apparatusas claimed in claim 75, wherein the developing liquid has the developingsubstance dispersed in a content of between 5% and 40% and has viscosityof between 100 mPa·s and 10,000 mPa·s.
 77. The apparatus as claimed inclaim 75, wherein said content sensing means comprises content signaloutputting means and content calculating means; said content signaloutputting means comprising: film forming means for causing thedeveloping liquid in said liquid storing section to form a film having athickness slope; light emitting means for emitting light toward the filmsuch that said light is transmitted through said film in a direction ofthickness; signal outputting means for outputting a signalrepresentative of a quantity of light incident thereto via the film; andshifting means for shifting a position of the film to which the light isincident in a direction of the thickness slope; wherein said contentcalculating means integrates a continuous output of said signaloutputting means received over a preselected period of time andcalculates, based on a result of integration, a content of thedeveloping substance of the developing liquid stored in said liquidstoring section.
 78. The apparatus as claimed in claim 77, furthercomprising liquid level sensing means for sensing a liquid level in saidliquid storing section, wherein the control agent is fed to said liquidstoring section in accordance with an output of said liquid levelsensing means.
 79. The apparatus as claimed in claim 78, wherein saidliquid level sensing means comprising: a magnetic force generatingmember; magnetic force sensing means for sensing a magnetic force ofsaid magnetic force generating member; floats movable in an up-and-downdirection in accordance with the liquid level in said liquid storingsection; and an elongate, ring support member supporting said floats atopposite ends thereof, supporting either one of said magnetic forcegenerating member and said magnetic force sensing means at a positionintermediate between said opposite ends, and supporting rings betweensaid position and said opposite ends; and a plurality of guide rodsrespectively inserted in said rings for guiding said floats in theup-and-down direction; wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 80. The apparatus as claimed in claim 79, wherein the developingliquid has the developing substance dispersed in a content of between 5%and 40% and has viscosity of between 100 mPa·s and 10,000 mPa·s.
 81. Theapparatus as claimed in claim 75, further comprising liquid levelsensing means for sensing a liquid level in said liquid storing section,wherein the control agent is fed to said liquid storing section inaccordance with an output of said liquid level sensing means.
 82. Theapparatus as claimed in claim 81, wherein said liquid level sensingmeans comprising: a magnetic force generating member; magnetic forcesensing means for sensing a magnetic force of said magnetic forcegenerating member; floats movable in an up-and-down direction inaccordance with the liquid level in said liquid storing section; and anelongate, ring support member supporting said floats at opposite endsthereof, supporting either one of said magnetic force generating memberand said magnetic force sensing means at a position intermediate betweensaid opposite ends, and supporting rings between said position and saidopposite ends; and a plurality of guide rods respectively inserted insaid rings for guiding said floats in the up-and-down direction; whereina distance between said magnetic force generating member and saidmagnetic force sensing means varies in accordance with a movement ofsaid floats, allowing the liquid level to be determined on the basis ofan output of said magnetic force sensing means.
 83. The apparatus asclaimed in claim 82, wherein the developing liquid has the developingsubstance dispersed in a content of between 5% and 40% and has viscosityof between 100 mPa·s and 10,000 mPa·s.
 84. An image forming apparatuscomprising: an image carrier for forming a latent image thereon; aliquid storing section for storing a developing liquid consisting of adeveloping substance and a carrier liquid; a developer carrier fordepositing the developing liquid fed from said liquid storing sectionthereon and causing said developing liquid to deposit on the latentimage to thereby develop said latent image; first cleaning means forremoving the developing liquid left on said image carrier afterdevelopment; content sensing means for sensing a content of thedeveloping substance of the developing liquid stored in said liquidstoring section; content controlling means for comparing an output ofsaid content sensing means and a preselected target content and feedinga control agent to said liquid storing section in accordance with aresult of comparison to thereby control a content of the developingsubstance; image density calculating means for calculating image densityof a developed image; a first residual liquid storing section forstoring the developing liquid removed by said first cleaning means;bypass conveying means for conveying the developing liquid from saidcleaning means to said liquid storing section by bypassing said firstresidual liquid storing section; and bypass controlling means forcontrolling said bypass conveying means in accordance with image densitycalculated by said image density calculating means.
 85. The apparatus asclaimed in claim 84, wherein the developing liquid has the developingsubstance dispersed in a content of between 5% and 40% and has viscosityof between 100 mPa·s and 10,000 mPa·s.
 86. The apparatus as claimed inclaim 84, wherein the developing liquid stored in said first residualliquid storing section is used as a control agent for controlling thecontent of the developing substance of the developing liquid stored insaid liquid storing section.
 87. The apparatus as claimed in claim 86,further comprising: transferring means for transferring a developedimage from said image carrier to an intermediate image transfer body andthen from said intermediate image transfer body to a recording medium;second cleaning means for removing the developing liquid left on saidimage carrier after image transfer; a second residual liquid storingsection for storing the developing liquid removed by said secondcleaning means; wherein a mixture of the developing liquid stored insaid first residual liquid storing section and the developing liquidstored in said second residual liquid storing section is used as thecontrol agent.
 88. The apparatus as claimed in claim 87, wherein acarrier liquid is used as the control agent.
 89. The apparatus asclaimed in claim 88, wherein the developing liquid stored in said secondresidual liquid storing section is used prior to the carrier liquid. 90.The apparatus as claimed in claim 89, wherein a developing liquid whosedeveloping substance has a content higher than a target content is usedas the control agent.
 91. The apparatus as claimed in claim 90, whereinthe target content is higher than a standard content, which is a contentdesirable for development, within a range that maintains a differencebetween development density derived from the developing density havingthe target content and development density derived from a developingliquid having said target content unobservable by eye.
 92. The apparatusas claimed in claim 91, further comprising storing means for storing aresult of calculation output from said image density calculating meansand the target density, wherein said target density stored in saidstoring means is updated in accordance with a plurality of results ofcalculation written in said storing means.
 93. The apparatus as claimedin claim 84, wherein said content sensing means comprises content signaloutputting means and content calculating means; said content signaloutputting means comprising: film forming means for causing thedeveloping liquid in said liquid storing section to form a film having athickness slope; light emitting means for emitting light toward the filmsuch that said light is transmitted through said film in a direction ofthickness; signal outputting means for outputting a signalrepresentative of a quantity of light incident thereto via the film; andshifting means for shifting a position of the film to which the light isincident in a direction of the thickness slope; wherein said contentcalculating means integrates a continuous output of said signaloutputting means received over a preselected period of time andcalculates, based on a result of integration, a content of thedeveloping substance of the developing liquid stored in said liquidstoring section.
 94. The apparatus as claimed in claim 93, furthercomprising liquid level sensing means for sensing a liquid level in saidliquid storing section, wherein the control agent is fed to said liquidstoring section in accordance with an output of said liquid levelsensing means.
 95. The apparatus as claimed in claim 94, wherein saidliquid level sensing means comprising: a magnetic force generatingmember; magnetic force sensing means for sensing a magnetic force ofsaid magnetic force generating member; floats movable in an up-and-downdirection in accordance with the liquid level in said liquid storingsection; and an elongate, ring support member supporting said floats atopposite ends thereof, supporting either one of said magnetic forcegenerating member and said magnetic force sensing means at a positionintermediate between said opposite ends, and supporting rings betweensaid position and said opposite ends; and a plurality of guide rodsrespectively inserted in said rings for guiding said floats in theup-and-down direction; wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 96. The apparatus as claimed in claim 95, wherein the developingliquid has the developing substance dispersed in a content of between 5%and 40% and has viscosity of between 100 mPa·s and 10,000 mPa·s.
 97. Theapparatus as claimed in claim 84, further comprising liquid levelsensing means for sensing a liquid level in said liquid storing section,wherein the control agent is fed to said liquid storing section inaccordance with an output of said liquid level sensing means.
 98. Theapparatus as claimed in claim 97, wherein said liquid level sensingmeans comprising: a magnetic force generating member; magnetic forcesensing means for sensing a magnetic force of said magnetic forcegenerating member; floats movable in an up-and-down direction inaccordance with the liquid level in said liquid storing section; and anelongate, ring support member supporting said floats at opposite endsthereof, supporting either one of said magnetic force generating memberand said magnetic force sensing means at a position intermediate betweensaid opposite ends, and supporting rings between said position and saidopposite ends; and a plurality of guide rods respectively inserted insaid rings for guiding said floats in the up-and-down direction; whereina distance between said magnetic force generating member and saidmagnetic force sensing means varies in accordance with a movement ofsaid floats, allowing the liquid level to be determined on the basis ofan output of said magnetic force sensing means.
 99. The apparatus asclaimed in claim 98, wherein the developing liquid has the developingsubstance dispersed in a content of between 5% and 40% and has viscosityof between 100 mPa·s and 10,000 mPa·s.
 100. An image forming apparatuscomprising: an image carrier for forming a latent image thereon; imageforming means for forming a latent image on said image carrier; a liquidstoring section for storing a developing liquid consisting of adeveloping substance and a carrier liquid; a developer carrier fordepositing the developing liquid fed from said liquid storing sectionthereon and causing said developing liquid to deposit on the latentimage to thereby develop said latent image; first cleaning means forremoving the developing liquid left on said image carrier afterdevelopment; first conveying means for conveying the developing liquidfrom said first cleaning means to said liquid storing section; contentsensing means for sensing a content of the developing substance of thedeveloping liquid stored in said liquid storing section; contentcontrolling means for comparing an output of said content sensing meansand a preselected target content and feeding a control agent to saidliquid storing section in accordance with a result of comparison tothereby control a content of the developing substance; and image densitycalculating means for calculating density of a developed image; whereina timing for starting forming the latent image on said image carrier isdetermined in accordance with image density output from said imagedensity calculating means.
 101. The apparatus as claimed in claim 100,wherein the developing liquid has the developing substance dispersed ina content of between 5% and 40% and has viscosity of between 100 mPa·sand 10,000 mPa·s.
 102. The apparatus as claimed in claim 100, furthercomprising: transferring means for transferring the developed image fromsaid image carrier to an intermediate image transfer body and then fromsaid intermediate image transfer body to a recording medium; secondcleaning means for removing the developing liquid from said imagecarrier after image transfer; and second conveying means for conveyingthe developing liquid from said second cleaning means to said liquidstoring section.
 103. The apparatus as claimed in claim 102, whereinsaid content sensing means comprises content signal outputting means andcontent calculating means; said content signal outputting meanscomprising: film forming means for causing the developing liquid in saidliquid storing section to form a film having a thickness slope; lightemitting means for emitting light toward the film such that said lightis transmitted through said film in a direction of thickness; signaloutputting means for outputting a signal representative of a quantity oflight incident thereto via the film; and shifting means for shifting aposition of the film to which the light is incident in a direction ofthe thickness slope; wherein said content calculating means integratesoutputs of said signal outputting means received over a preselectedperiod of time and calculates, based on a result of integration, acontent of the developing substance of the developing liquid stored insaid liquid storing section.
 104. The apparatus as claimed in claim 103,further comprising liquid level sensing means for sensing a liquid levelin said liquid storing section, wherein the control agent is fed to saidliquid storing section in accordance with an output of said liquid levelsensing means.
 105. The apparatus as claimed in claim 104, wherein saidliquid level sensing means comprising: a magnetic force generatingmember; magnetic force sensing means for sensing a magnetic force ofsaid magnetic force generating member; floats movable in an up-and-downdirection in accordance with the liquid level in said liquid storingsection; and an elongate, ring support member supporting said floats atopposite ends thereof, supporting either one of said magnetic forcegenerating member and said magnetic force sensing means at a positionintermediate between said opposite ends, and supporting rings betweensaid position and said opposite ends; and a plurality of guide rodsrespectively inserted in said rings for guiding said floats in theup-and-down direction; wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 106. The apparatus as claimed in claim 105, wherein thedeveloping liquid has the developing substance dispersed in a content ofbetween 5% and 40% and has viscosity of between 100 mPa·s and 10,000mPa·s.
 107. The apparatus as claimed in claim 100, wherein said contentsensing means comprises content signal outputting means and contentcalculating means; said content signal outputting means comprising: filmforming means for causing the developing liquid in said liquid storingsection to form a film having a thickness slope; light emitting meansfor emitting light toward the film such that said light is transmittedthrough said film in a direction of thickness; signal outputting meansfor outputting a signal representative of a quantity of light incidentthereto via the film; and shifting means for shifting a position of thefilm to which the light is incident in a direction of the thicknessslope; wherein said content calculating means integrates outputs of saidsignal outputting means received over a preselected period of time andcalculates, based on a result of integration, a content of thedeveloping substance of the developing liquid stored in said liquidstoring section.
 108. The apparatus as claimed in claim 107, furthercomprising liquid level sensing means for sensing a liquid level in saidliquid storing section, wherein the control agent is fed to said liquidstoring section in accordance with an output of said liquid levelsensing means.
 109. The apparatus as claimed in claim 108, wherein saidliquid level sensing means comprising: a magnetic force generatingmember; magnetic force sensing means for sensing a magnetic force ofsaid magnetic force generating member; floats movable in an up-and-downdirection in accordance with the liquid level in said liquid storingsection; and an elongate, ring support member supporting said floats atopposite ends thereof, supporting either one of said magnetic forcegenerating member and said magnetic force sensing means at a positionintermediate between said opposite ends, and supporting rings betweensaid position and said opposite ends; and a plurality of guide rodsrespectively inserted in said rings for guiding said floats in theup-and-down direction; wherein a distance between said magnetic forcegenerating member and said magnetic force sensing means varies inaccordance with a movement of said floats, allowing the liquid level tobe determined on the basis of an output of said magnetic force sensingmeans.
 110. The apparatus as claimed in claim 109, wherein thedeveloping liquid has the developing substance dispersed in a content ofbetween 5% and 40% and has viscosity of between 100 mPa·s and 10,000mPa·s.
 111. The apparatus as claimed in claim 100, further comprisingliquid level sensing means for sensing a liquid level in said liquidstoring section, wherein the control agent is fed to said liquid storingsection in accordance with an output of said liquid level sensing means.112. The apparatus as claimed in claim 111, wherein said liquid levelsensing means comprising: a magnetic force generating member; magneticforce sensing means for sensing a magnetic force of said magnetic forcegenerating member; floats movable in an up-and-down direction inaccordance with the liquid level in said liquid storing section; and anelongate, ring support member supporting said floats at opposite endsthereof, supporting either one of said magnetic force generating memberand said magnetic force sensing means at a position intermediate betweensaid opposite ends, and supporting rings between said position and saidopposite ends; and a plurality of guide rods respectively inserted insaid rings for guiding said floats in the up-and-down direction; whereina distance between said magnetic force generating member and saidmagnetic force sensing means varies in accordance with a movement ofsaid floats, allowing the liquid level to be determined on the basis ofan output of said magnetic force sensing means.
 113. The apparatus asclaimed in claim 112, wherein the developing liquid has the developingsubstance dispersed in a content of between 5% and 40% and has viscosityof between 100 mPa·s and 10,000 mPa·s.